[CODE HEALTH] clean up nostd::variant access in otlp exporters

[dbarker]

Fixes # (issue)

Changes

• clean up use of nostd::holds_alternative/nostd::get if/else pattern
  • replace with nostd::visit in otlp AnyValue attribute utils
  • replace with nostd::get_if in otlp http client and otlp metric utils
• Add tests for OtlpPopulateAttributeUtils
• minor updates to OtlpMetricUtils to reduce data point copies and use const
• update conversion of uint64 attribute values to match the standard (map to string on overflow of int).
  • See: https://opentelemetry.io/docs/specs/otel/common/attribute-type-mapping/#integer-values

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

❌ Patch coverage is 96.50350% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.08%. Comparing base (0668d64) to head (294a38b).

Files with missing lines	Patch %	Lines
exporters/otlp/src/otlp_metric_utils.cc	88.89%	4 Missing ⚠️
exporters/otlp/src/otlp_http_client.cc	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3973      +/-   ##
==========================================
+ Coverage   82.04%   82.08%   +0.05%     
==========================================
  Files         385      385              
  Lines       15895    15916      +21     
==========================================
+ Hits        13039    13063      +24     
+ Misses       2856     2853       -3     

Files with missing lines	Coverage Δ
...xporters/otlp/src/otlp_populate_attribute_utils.cc	98.00% <100.00%> (+3.31%)	⬆️
exporters/otlp/src/otlp_http_client.cc	67.64% <75.00%> (+0.07%)	⬆️
exporters/otlp/src/otlp_metric_utils.cc	90.53% <88.89%> (-1.49%)	⬇️

... and 1 file with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.