Allow exporting multiple times for unit tests of OTLP HTTP exporters

[owent]

Fixes #3721

Changes

• Allow retryable exports or split them into two batch jobs for OTLP HTTP exporters.

For significant contributions please make sure you have completed the following items:

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

[Copilot]

Pull request overview

This PR modifies OTLP HTTP exporter tests to support multiple export attempts in unit tests, addressing issue #3721. The changes enable tests to handle retryable exports or split batch jobs.

Key Changes

• Changed mock expectations from .WillOnce() to .WillRepeatedly() to allow multiple export invocations
• Added early-return guards to handle empty payloads gracefully
• Introduced counter validation to ensure at least one successful export occurred

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 14 comments.

File	Description
exporters/otlp/test/otlp_http_metric_exporter_test.cc	Updated 6 test methods to support retryable metric exports with counter validation
exporters/otlp/test/otlp_http_log_record_exporter_test.cc	Updated 4 test methods to support retryable log exports with counter validation
exporters/otlp/test/otlp_http_exporter_test.cc	Updated 4 test methods to support retryable trace exports with counter validation

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.95%. Comparing base (9de3493) to head (ab78ffb).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3771      +/-   ##
==========================================
+ Coverage   89.93%   89.95%   +0.02%     
==========================================
  Files         225      225              
  Lines        7158     7158              
==========================================
+ Hits         6437     6438       +1     
+ Misses        721      720       -1     

see 1 file with indirect coverage changes

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