[PROMETHEUS] Upgrade otelcpp to 1.29, add prometheus file exporter by owent · Pull Request #646 · open-telemetry/opentelemetry-cpp-contrib

owent · 2026-08-05T07:18:59Z

Fixes #713

Fixes some thread-safety problems.
Implement prometheus file exporter.
Upgrade otelcpp to 1.28.

Copilot

Pull request overview

This PR updates the Prometheus exporter module to OpenTelemetry C++ 1.28 and adds a new Prometheus file exporter implementation alongside the existing push exporter.

Changes:

Bump Prometheus module versions/references from 1.19.0 to 1.28.0 (CMake/Bazel/GitHub Actions/vcpkg).
Add a new Prometheus file exporter (implementation, options, factory) and wire it into CMake + Bazel builds.
Extend Prometheus push exporter options/translation path with populate_target_info and without_otel_scope flags.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
exporters/prometheus/vcpkg.json	Adds vcpkg manifest for the Prometheus contrib module (now needs metadata aligned with push + file exporters).
exporters/prometheus/tools.cmake	Introduces CMake helpers for import/export visibility; contains Windows attribute and SunPro detection issues.
exporters/prometheus/src/push_exporter.cc	Plumbs new translation options through push exporter collection/translation.
exporters/prometheus/src/push_exporter_factory.cc	Exports the factory `Create` symbol via the new API macro.
exporters/prometheus/src/file_exporter.cc	Adds the new rotating-file exporter implementation (includes time/path formatting, background flush thread).
exporters/prometheus/src/file_exporter_factory.cc	Adds factory entry point for constructing the new file exporter.
exporters/prometheus/MODULE.bazel	Updates Bazel module + dependency versions to 1.28.0.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter.h	Minor formatting-only change.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter_options.h	Adds new options + API visibility macro definition.
exporters/prometheus/include/opentelemetry/exporters/prometheus/push_exporter_factory.h	Exposes factory `Create` with API macro and includes options header.
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter.h	Adds public header for the new file exporter (needs API export annotation).
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter_options.h	Adds options + API visibility macro definition for file exporter.
exporters/prometheus/include/opentelemetry/exporters/prometheus/file_exporter_factory.h	Adds public factory header for the file exporter.
exporters/prometheus/CMakeLists.txt	Updates project version, adds new library target, and adds import/export macro wiring.
exporters/prometheus/BUILD	Adds Bazel `cc_library` target for the file exporter.
.github/workflows/prometheus.yml	Updates CI to test against opentelemetry-cpp v1.28.0.

Suppressed comments (3)

exporters/prometheus/tools.cmake:44

Same issue as the export path: __attribute__((__dllimport__)) is not the standard GCC/Clang attribute spelling. This likely results in no import decoration on Windows with MinGW/Clang.

  if(CMAKE_CXX_COMPILER_ID MATCHES "GNU|Clang|AppleClang|Intel|XL|XLClang")
    if(CMAKE_SYSTEM_NAME STREQUAL "Windows")
      set(${OUTPUT_VARNAME}
          "__attribute__((__dllimport__))"
          PARENT_SCOPE)

exporters/prometheus/src/file_exporter.cc:693

~PrometheusFileBackend() joins the background flush thread but never sets is_shutdown to stop it. The thread can run for up to 1 minute before exiting (idle timeout), so destroying the exporter can block for a long time if Shutdown() wasn’t called.

  ~PrometheusFileBackend()
  {
    if (file_)
    {
      file_->background_thread_waker_cv.notify_all();
      std::unique_ptr<std::thread> background_flush_thread;

exporters/prometheus/src/file_exporter.cc:1089

CheckUpdate() updates rotate_index and calls ResetLogFile() without holding file_lock, even though ResetLogFile() explicitly assumes it is called under lock. This is a data race if AddMetricData() can be invoked concurrently, and can also conflict with OpenLogFile()/RotateLog() which use the same state under file_lock.

    // Reset rotate index when directory changes
    if (new_dir != old_dir)
    {
      file_->rotate_index = 0;
    }

    ResetLogFile();

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owent · 2026-08-05T15:18:36Z

+class PrometheusFileExporter : public ::opentelemetry::sdk::metrics::PushMetricExporter
+{


It's not public header

owent · 2026-08-09T13:30:52Z

Sorry for it's be a long time to update this package. Could you please review it again when you have time? @ThomsonTan @esigo @lalitb

proost

Maintainers looks very busy, so i'd like to help out.

Can you add test cases for file exporter?

proost · 2026-08-12T10:58:16Z

+      return 0;
+    }
+
+    file.seekg(std::ios::end);


file.seekg(0, std::ios::end)

Thanks, unit tests are added.

proost · 2026-08-12T11:05:35Z

+    std::string new_dir = FileSystemUtil::DirName(new_file_path);
+    std::string old_dir = FileSystemUtil::DirName(old_file_path);
+
+    // Reset rotate index when directory changes


Looks like we should hold lock in here to call ResetLogFile. Am i correct?

It's not required to be very precise in file size calculation. So I changed all the variables in ResetLogFile tobe atomic.And no lock is reauired neither.

proost · 2026-08-12T11:18:43Z

+    file_->flushed_metric_family_count.store(0);
+  }
+
+  ~PrometheusFileBackend()


Are there no need to set "is_shutdown" to true?

Thanks, is_shutdown is set now.

proost · 2026-08-19T12:28:29Z

+        std::lock_guard<std::mutex> lock_guard{file_->background_thread_lock};
+        if (!file_->background_flush_thread)
+        {
+          break;


non-blocking; no background flush thread and "is_shutdown" is true, can we return earlier right?

Sorry, I'm not sure I follow. When there's been no IO for a while, the background thread exits on its own, so it shouldn't matter whether it's explicitly shut down or not.

owent · 2026-09-15T03:08:50Z

@proost Could you please review this PR again when you have time? Thanks.

proost

Thanks!

proost · 2026-09-15T12:18:12Z

+  bool ForceFlush(std::chrono::microseconds timeout) noexcept
+  {
+    std::chrono::microseconds wait_interval = timeout / 256;
+    if (wait_interval <= std::chrono::microseconds{0})


non-blocking; Maybe guarding unrealistic too large value too?

Max wait interval is added. And I also upgrade the upstream otel-cpp to 1.29. Could you please review again when you have time? @proost

proost

LGTM

thompson-tomo · 2026-09-21T15:27:28Z

-          bazel --output_user_root=$HOME/.cache/bazel build --copt=-DENABLE_TEST --@io_opentelemetry_cpp//api:with_abseil //...
-          bazel --output_user_root=$HOME/.cache/bazel test --copt=-DENABLE_TEST --@io_opentelemetry_cpp//api:with_abseil //...
+          bazel --output_user_root=$HOME/.cache/bazel build --@io_opentelemetry_cpp//api:with_abseil //...
+          bazel --output_user_root=$HOME/.cache/bazel test --@io_opentelemetry_cpp//api:with_abseil //...


It looks like to me that the tests are now not actually being run especially when comparing the logs from this pr to https://github.com/open-telemetry/opentelemetry-cpp-contrib/actions/runs/35523736398/job/106334223469#step:4:1

thompson-tomo · 2026-09-22T03:39:04Z

@owent could you please take a look at resolving the conflicts.

^ Conflicts: ^ exporters/prometheus/MODULE.bazel Update bazel modules Fixes bazel and cmake support Fixes prometheus linking Fixes document Fixes compatibility Fixes compatibility of localtime_s usage Make all variables in ResetLogFile atomic Add unit test for PrometheusFileExporter Merge concunrrency fixes from <open-telemetry/opentelemetry-cpp#4365> Update otel-cpp to 1.29.0 Limit max value for wait interval ^ Conflicts: ^ exporters/prometheus/MODULE.bazel ^ Conflicts: ^ .github/workflows/prometheus.yml ^ exporters/prometheus/.bazelversion ^ exporters/prometheus/CMakeLists.txt ^ exporters/prometheus/MODULE.bazel ^ exporters/prometheus/repository.bzl

Fixes test environment

owent · 2026-09-22T09:16:01Z

@owent could you please take a look at resolving the conflicts.

Conflicts are resolved, unit tests are enabled now.

thompson-tomo · 2026-09-22T09:23:30Z

@@ -44,17 +38,33 @@ jobs:
          cache-name: bazel_cache
        with:
          path: /home/runner/.cache/bazel
-          key: bazel_${{ matrix.os }}
+          key: bazel_linux
      - name: run build
        working-directory: otel_cpp_contrib/exporters/prometheus
        run: |
-          bazel --output_user_root=$HOME/.cache/bazel build --copt=-DENABLE_TEST --@io_opentelemetry_cpp//api:with_abseil //...
+          bazel --output_user_root=$HOME/.cache/bazel build --@io_opentelemetry_cpp//api:with_abseil //...
+          bazel --output_user_root=$HOME/.cache/bazel test --test_output=all --@io_opentelemetry_cpp//api:with_abseil //...
+  prometheus_bazel_osx:
+    name: Bazel on MacOS
+    runs-on: macos-latest
+    steps:
+      - name: checkout otel contrib prometheus
+        uses: actions/checkout@3d3c42e5aac5ba805825da76410c181273ba90b1 # v7.0.1
+        with:
+          persist-credentials: false
+          path: "otel_cpp_contrib"
+      - name: Mount Bazel Cache
+        uses: actions/cache@55cc8345863c7cc4c66a329aec7e433d2d1c52a9 # v6.1.0
+        env:
+          cache-name: bazel_cache
+        with:
+          path: /Users/runner/.cache/bazel
+          key: bazel_osx
      - name: run tests
-        if: ${{ !startsWith(matrix.os, 'ubuntu') }}
        working-directory: otel_cpp_contrib/exporters/prometheus
        run: |
-          bazel --output_user_root=$HOME/.cache/bazel test --copt=-DENABLE_TEST --@io_opentelemetry_cpp//api:with_abseil //...
-
+          bazel --output_user_root=$HOME/.cache/bazel build --@io_opentelemetry_cpp//api:with_abseil //...
+          bazel --output_user_root=$HOME/.cache/bazel test --test_output=all --@io_opentelemetry_cpp//api:with_abseil //...


@owent can you put this back to running as a matrix

@owent

I was about to ask about this too: what is the rationale for going back to 2 separate ci tasks ?

A matrix is easier to extend and avoids duplication.

thompson-tomo · 2026-09-22T09:25:26Z

+          # Install both features before OpenTelemetry can build a pull-only dependency.
+          vcpkg install "prometheus-cpp[pull,push]" --triplet=x64-linux --classic


This shouldn't be needed as apt-packages install it without vcpkg

thompson-tomo · 2026-09-22T09:28:04Z

          cd "$GITHUB_WORKSPACE/otel_cpp/build_jobs_ci"
-          cmake .. \
-            -G Ninja \
-            -DWITH_PROMETHEUS=ON \
-            -DWITH_METRICS_PREVIEW=OFF \
-            -DBUILD_TESTING=OFF -DWITH_EXAMPLES=OFF  -DOPENTELEMETRY_INSTALL=ON "-DWITH_FUNC_TESTS=OFF"
-          cmake --build . -j$(nproc)
-          cmake --install . --prefix="${GITHUB_WORKSPACE}/sandbox"
+          cmake .. -DOTELCPP_WITH_PROMETHEUS=ON -DCMAKE_BUILD_TYPE=Debug \
+            -DVCPKG_TARGET_TRIPLET=x64-linux \
+            "-DCMAKE_INSTALL_PREFIX=$HOME/prebuilt-otel" \
+            "-DCMAKE_TOOLCHAIN_FILE=$VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake" \
+            "-DBUILD_TESTING=OFF" "-DOTELCPP_WITH_EXAMPLES=OFF" "-DOTELCPP_WITH_FUNC_TESTS=OFF"
+          cmake --build . -j --config Debug || cmake --build . -j2 --config Debug || cmake --build . --config Debug
+          cmake --install . --prefix "$HOME/prebuilt-otel" --config Debug
          mkdir -p "$GITHUB_WORKSPACE/otel_cpp_contrib/exporters/prometheus/build_jobs_ci"
          cd "$GITHUB_WORKSPACE/otel_cpp_contrib/exporters/prometheus/build_jobs_ci"
-          cmake .. \
-            -G Ninja \
-            -DCMAKE_PREFIX_PATH="${GITHUB_WORKSPACE}/sandbox" \
-            -DCMAKE_BUILD_TYPE=Debug
-          cmake --build . -j$(nproc)
+          cmake .. "-DCMAKE_PREFIX_PATH=$HOME/prebuilt-otel" -DCMAKE_BUILD_TYPE=Debug \
+            -DVCPKG_TARGET_TRIPLET=x64-linux \
+            "-DCMAKE_TOOLCHAIN_FILE=$VCPKG_INSTALLATION_ROOT/scripts/buildsystems/vcpkg.cmake" \
+            "-DVCPKG_MANIFEST_MODE=OFF"
+          cmake --build . -j --config Debug || cmake --build . -j2 --config Debug || cmake --build . --config Debug


Let's revert this so that we can maintain the native cmake ci.

If a vcpkg ci is wanted, let's add this as a dedicated job.

thompson-tomo · 2026-09-22T09:33:08Z

-bazel_dep(name = "abseil-cpp", version = "20260526.0", repo_name = "com_google_absl")
+bazel_dep(name = "abseil-cpp", version = "20260107.1", repo_name = "com_google_absl")
 bazel_dep(name = "opentelemetry-cpp", version = "1.29.0", repo_name = "io_opentelemetry_cpp")
-bazel_dep(name = "prometheus-cpp", version = "1.3.0.bcr.3", repo_name = "com_github_jupp0r_prometheus_cpp")
+bazel_dep(name = "prometheus-cpp", version = "1.3.0.bcr.2", repo_name = "com_github_jupp0r_prometheus_cpp")


Why are we downgrading

Looks like a bad merge.

@owent

Please fix.

thompson-tomo · 2026-09-22T09:33:45Z

    name = "opentelemetry-cpp-contrib-prometheus",
-    version = "1.19.0",
+    version = "1.29.0",
+    compatibility_level = 0,


Suggested change

compatibility_level = 0,

@owent

Please remove compatibility_level. This is needed to upgrade bazel to 8.8.0.

thompson-tomo · 2026-09-22T09:35:01Z

-bazel_dep(name = "googletest", version = "1.18.0.bcr.1", dev_dependency = True, repo_name = "com_google_googletest")
+bazel_dep(name = "googletest", version = "1.17.0.bcr.2", dev_dependency = True, repo_name = "com_google_googletest")


Why are we downgrading?

@owent

Please fix.

marcalff · 2026-09-22T14:54:40Z

@owent could you please take a look at resolving the conflicts.

Conflicts are resolved, unit tests are enabled now.

Thanks, but it looks like there is some code rolled back during conflict resolution.

Please check, looking to merge this PR once makefiles and CI yaml files looks ok.

Copilot AI lite review requested due to automatic review settings August 5, 2026 07:19

owent requested a review from a team as a code owner August 5, 2026 07:19

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owent force-pushed the prometheus_exporters branch from 7b153bb to da9b374 Compare August 5, 2026 12:43

owent mentioned this pull request Aug 9, 2026

Update OtlpGrpcClientOptions to populate options for gRPC client sharing open-telemetry/opentelemetry-cpp#4249

Open

proost reviewed Aug 12, 2026

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owent force-pushed the prometheus_exporters branch from 436105b to ac28893 Compare August 17, 2026 06:21

proost reviewed Aug 19, 2026

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owent force-pushed the prometheus_exporters branch from ba10b0e to 4156383 Compare August 20, 2026 06:18

owent requested a review from proost September 15, 2026 03:07

proost approved these changes Sep 15, 2026

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owent force-pushed the prometheus_exporters branch 2 times, most recently from 692df86 to e7c574b Compare September 18, 2026 05:01

owent changed the title ~~[PROMETHEUS] Upgrade otelcpp to 1.28, add prometheus file exporter~~ [PROMETHEUS] Upgrade otelcpp to 1.29, add prometheus file exporter Sep 18, 2026

owent force-pushed the prometheus_exporters branch from e7c574b to d54d8a5 Compare September 18, 2026 06:15

proost approved these changes Sep 18, 2026

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thompson-tomo reviewed Sep 21, 2026

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owent force-pushed the prometheus_exporters branch from d54d8a5 to 91b903a Compare September 22, 2026 08:28

github-actions Bot added the exporter:prometheus Prometheus Exporter label Sep 22, 2026

otel-cpp and prometheus do not depend macro ENABLE_TEST now.

e71e9d2

Fixes test environment

owent force-pushed the prometheus_exporters branch from 91b903a to e71e9d2 Compare September 22, 2026 08:55

owent mentioned this pull request Sep 22, 2026

Prometheus tests are disabled on ubuntu #713

Open

Add more output for unit tests of prometheus

da607b4

thompson-tomo reviewed Sep 22, 2026

View reviewed changes

		class PrometheusFileExporter : public ::opentelemetry::sdk::metrics::PushMetricExporter
		{

		# Install both features before OpenTelemetry can build a pull-only dependency.
		vcpkg install "prometheus-cpp[pull,push]" --triplet=x64-linux --classic

		bazel_dep(name = "googletest", version = "1.18.0.bcr.1", dev_dependency = True, repo_name = "com_google_googletest")
		bazel_dep(name = "googletest", version = "1.17.0.bcr.2", dev_dependency = True, repo_name = "com_google_googletest")

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