feat: remove ifdef opentelemetry statements in handwritten libs

[mpeddada1]

This PR:

• Removes ifdef and associated else blocks for handwritten code.
• Removes the GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY declaration from build files.

[gemini-code-assist]

Summary of Changes

Hello @mpeddada1, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly simplifies the codebase by removing conditional compilation directives related to OpenTelemetry. Previously, sections of code were conditionally compiled based on the GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY macro. By removing these directives and the corresponding build file declarations, the PR ensures that OpenTelemetry tracing is consistently integrated across all handwritten libraries, reducing code complexity and maintenance overhead. This change implies that OpenTelemetry is now a standard, always-enabled feature within these components.

Highlights

• Code Simplification: Removed all instances of #ifdef GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY and associated #else blocks from handwritten code, streamlining the codebase.
• Build Configuration Update: Eliminated the GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY declaration from both Bazel (BUILD.bazel) and CMake (google_cloud_cpp_common.cmake) build files.

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[gemini-code-assist]

Code Review

This pull request removes the conditional compilation guards (#ifdef GOOGLE_CLOUD_CPP_HAVE_OPENTELEMETRY) for OpenTelemetry across the codebase. This is a significant and widespread change that makes OpenTelemetry a non-optional dependency. The changes look good and are consistent with the goal of the PR. I've found one minor issue in a CMake file where a no-op command can be removed. Otherwise, the refactoring is well-executed.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.94%. Comparing base (febba47) to head (a3367fa).
⚠️ Report is 1 commits behind head on prepare-for-v3.0.0.

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15838      +/-   ##
======================================================
- Coverage               92.96%   92.94%   -0.03%     
======================================================
  Files                    2455     2454       -1     
  Lines                  227501   227415      -86     
======================================================
- Hits                   211495   211369     -126     
- Misses                  16006    16046      +40     

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