[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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.18%. Comparing base (fef7bbb) to head (d03999c).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3973   +/-   ##
=======================================
  Coverage   90.18%   90.18%           
=======================================
  Files         230      230           
  Lines        7298     7298           
=======================================
  Hits         6581     6581           
  Misses        717      717           

🚀 New features to boost your workflow:

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