[OTLP/HTTP] Honor Retry-After header when retrying exports

[DCchoudhury15]

Fixes #4172

Changes

The OTLP/HTTP retry logic wasn't respecting the Retry-After response header. If a server returned a 429 or 503 along with Retry-After, the exporter would ignore that value and always use its configured exponential backoff instead.

This PR updates the retry behavior to honor the Retry-After header as defined in RFC 7231 §7.1.3. Both supported formats are handled:

• A delay in seconds (for example, Retry-After: 5)
• An HTTP-date (for example, Retry-After: Wed, 21 Oct 2015 07:28:00 GMT)

One implementation detail worth noting is that NextRetryTime() is called from the doRetrySessions polling loop after ReleaseResponse() has already cleared the response headers. Because of that, the parsed Retry-After value is cached in retry_after_time_point_ before ReleaseResponse() is called, and NextRetryTime() uses the cached value instead of trying to parse the headers again.

Unit tests are not included yet. I can add them directly to this PR
if the maintainers would like the plan would be to extend the existing
test server fixture with a /retry-after/ endpoint that sets the header,
and add two test cases to ext/test/http/curl_http_test.cc following
the pattern of the existing ExponentialBackoffRetry test: one for
Retry-After: <delay-seconds> and one for Retry-After: <HTTP-date>.
Just let me know and I will push them here.

Known limitation: The HTTP-date parser currently supports only the IMF-fixdate format. The two obsolete date formats (RFC 850 and asctime) are not supported. Since RFC 7231 specifies IMF-fixdate as the required format for senders, this should not be an issue in practice.

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[codecov]

Codecov Report

❌ Patch coverage is 14.28571% with 42 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.78%. Comparing base (65aae9c) to head (8cdef0a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
ext/src/http/client/curl/http_operation_curl.cc	14.29%	42 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4186      +/-   ##
==========================================
- Coverage   82.99%   82.78%   -0.20%     
==========================================
  Files         406      406              
  Lines       17260    17308      +48     
==========================================
+ Hits        14323    14327       +4     
- Misses       2937     2981      +44     

Files with missing lines	Coverage Δ
...lemetry/ext/http/client/curl/http_operation_curl.h	90.91% <ø> (ø)
ext/src/http/client/curl/http_operation_curl.cc	55.17% <14.29%> (-3.57%)	⬇️

... and 1 file with indirect coverage changes

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