[TEST] Make OTLP gRPC functional teardown deterministic

[yigitcan-ozturk]

Contributes to #4489.

Problem

func_otlp_grpc intermittently exits with SIGSEGV / heap-corruption signatures during TLS failure cases after the normal ctest suite has already passed. The failure still reproduces with gRPC 1.83.1, so the earlier OpenSSL NO_ATEXIT explanation is not sufficient on its own.

Hypothesis

The functional binary is short-lived. C++ channel wrappers release their own gRPC runtime references as the OpenTelemetry exporter/provider is destroyed, and the last release can initiate asynchronous gRPC global teardown immediately before main() returns. TLS failure cases leave enough EventEngine/background teardown work to expose that race intermittently.

Change

• hold one explicit grpc_init() reference for the functional-test invocation;
• after the test has destroyed its OpenTelemetry/gRPC objects, release that final reference with grpc_shutdown_blocking();
• keep the change scoped to the functional test binary; no exporter/runtime production behavior changes.

This is intentionally a draft while CI and maintainer review validate whether the teardown barrier removes the intermittent failure. If the hypothesis is wrong, I will not promote this as a fix.

Validation target

The important signal is repeated maintainer-mode CMake coverage of the TLS functional cases, especially the paths that have previously failed with exit 134/139.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.46%. Comparing base (bcd4f2d) to head (ea78fa2).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4541      +/-   ##
==========================================
- Coverage   83.47%   83.46%   -0.01%     
==========================================
  Files         521      522       +1     
  Lines       20380    20440      +60     
==========================================
+ Hits        17011    17059      +48     
- Misses       3369     3381      +12     

see 10 files with indirect coverage changes

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