Add multimodal preprocessing metrics

[CUHKSZzxy]

Summary

• Add multimodal preprocessing metrics for VLM requests, including request/item counters, total preprocessing latency, per-stage latency, per-request item count, and preprocessing failure counters.
• Record multimodal metrics through the existing metrics processor and Prometheus logger path.
• Export multimodal detailed metrics by default whenever the metrics system is enabled; no extra CLI flag is required.

Validation

• Focused multimodal metrics tests passed.
• Existing metrics logger tests passed.
• Syntax checks for touched metrics, serve, CLI, and message modules passed.
• API server help was checked to confirm only the existing metrics switch remains.

Benchmark

• Local OpenAI-compatible API macrobenchmarks compared this branch with main on a VLM image workload.
• No throughput regression was observed in the large image payload run; the branch was within expected run-to-run variance.

Terminal Log

When a logging interval includes multimodal preprocessing, the terminal stats line now appends the average multimodal preprocessing latency:

[2026-06-01 12:18:04 Engine 000] Avg thr (in/out): 0.0 / 0.0 tokens/s, Server (succeeded/failed/routed/waiting): 0 / 1 / 0 / 0, Engine (running/waiting): 0 / 0, KV cache: 0.0%, Avg MM preprocess: 0.051 s/req,

Assistance

Assisted with Codex + GPT-5.5 xHigh Fast

[Copilot]

Pull request overview

This PR adds end-to-end metrics for multimodal (VLM) prompt preprocessing, wiring new stats collection from request processing through the existing metrics processor and Prometheus/console loggers, and documenting the exported metrics.

Changes:

• Introduces MultimodalStats to track multimodal item counts, per-stage/total preprocessing latency, and failures.
• Instruments MultimodalProcessor and AsyncEngine.generate() to collect and emit multimodal preprocessing stats via metrics_processor.
• Extends metrics loggers (Prometheus + periodic console logger) and updates EN/ZH metrics documentation; adds targeted tests for the new multimodal metrics.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
tests/test_lmdeploy/test_metrics_multimodal.py	Adds unit tests covering multimodal stats behavior and Prometheus emission.
lmdeploy/serve/processors/multimodal.py	Instruments multimodal parsing + VLM preprocessing stages and threadsafe stats updates.
lmdeploy/serve/core/async_engine.py	Creates/records per-request multimodal stats in the request path.
lmdeploy/metrics/stats.py	Adds MultimodalStats for multimodal preprocessing accounting.
lmdeploy/metrics/metrics_processor.py	Adds record_multimodal() to emit multimodal stats through configured loggers.
lmdeploy/metrics/loggers.py	Implements multimodal metric export for Prometheus and aggregates for console logging.
docs/zh_cn/advance/metrics.md	Documents newly exported multimodal preprocessing metrics (CN).
docs/en/advance/metrics.md	Documents newly exported multimodal preprocessing metrics (EN).

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[CUHKSZzxy]

This is existing behavior from main: the stored value is a string, and Modality.eq supports comparing against that string.