[BUILD] Allow user to enable prometheus-cpp::push when using internal building script.

[owent]

This is a follow up work for open-telemetry/opentelemetry-cpp-contrib#646

Changes

• Add OTELCPP_WITH_PROMETHEUS_PUSH which allow to enable prometheus-cpp::push when reusing the build script from otel-cpp.

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.65%. Comparing base (754928d) to head (685baf7).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4653      +/-   ##
==========================================
- Coverage   86.66%   86.65%   -0.00%     
==========================================
  Files         525      525              
  Lines       20481    20481              
==========================================
- Hits        17748    17746       -2     
- Misses       2733     2735       +2     

see 1 file with indirect coverage changes

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[marcalff]

LGTM, thanks for the fix.

[Copilot]

Copilot review overview

🔵 Needs a closer look

It changes third-party dependency build configuration (FetchContent/ENABLE_PUSH) and adds a FATAL_ERROR path intended for external script consumers, which warrants human verification across build configurations that cannot be executed here.

Review effort: Balanced
Findings: None

What changed in this PR

This PR adds a build-time hook, OTELCPP_WITH_PROMETHEUS_PUSH, so that consumers who reuse opentelemetry-cpp's internal CMake scripts (e.g., the opentelemetry-cpp-contrib repo, continuing the work from contrib #646) can opt into building the prometheus-cpp::push component. Previously ENABLE_PUSH was hard-forced to OFF in cmake/prometheus-cpp.cmake, making the push library unavailable through the fetched/built prometheus-cpp path.

Changes:

• In cmake/prometheus-cpp.cmake, derive prometheus-cpp's ENABLE_PUSH from OTELCPP_WITH_PROMETHEUS_PUSH when defined (defaulting to OFF), disable IWYU/clang-tidy on the push target when enabled, add a FATAL_ERROR guard if the push target is unexpectedly built while the flag is off, and clean up the ENABLE_PUSH cache entry afterward.
• In ci/do_ci.sh, enable -DOTELCPP_WITH_PROMETHEUS_PUSH=ON in the cmake.maintainer.yaml.test target to exercise the new build path.

File	Description
cmake/​prometheus-cpp.cmake	Makes ENABLE_PUSH configurable via OTELCPP_WITH_PROMETHEUS_PUSH, sanitizes the push target's IWYU/clang-tidy properties, adds a safety guard, and unsets the cache var.
ci/​do_ci.sh	Enables the new push flag in the maintainer YAML test so the push build path is covered in CI.

Notes from investigation (no code comments warranted):

• The raw target name push used in set_target_properties(push ...) is correct — jupp0r/prometheus-cpp's push/CMakeLists.txt defines target push with alias prometheus-cpp::push, consistent with the existing core pull civetweb handling.
• ENABLE_PUSH is read at line 58 before being unset at line 67, so the deferred unset is intentional and correct. option(ENABLE_PUSH ...) inside prometheus-cpp won't override the forced cache value.
• The push library requires CURL::libcurl; in cmake.maintainer.yaml.test, curl is available because OTLP HTTP is enabled, so enabling push there is safe.
• OTELCPP_WITH_PROMETHEUS_PUSH is deliberately not registered via otelcpp_option_flag/printed in the STATUS summary; since opentelemetry-cpp itself does not consume the push exporter, registering it as a first-class user option could mislead, so this omission is reasonable rather than a convention violation.

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[lalitb]

nit - Could we make this job build Prometheus from source? It currently uses an already-installed version, so the new option is ignored and the code added here isn't tested.