[BUILD] Avoid dependency on protobuf from the OTLP HTTP metrics exporter header

[marcalff]

Fixes #2169

Changes

Simplify the header dependencies for file:

• exporters/otlp/include/opentelemetry/exporters/otlp/otlp_http_metric_exporter.h

The declaration of class OtlpHttpMetricExporter does not need to see code in otlp_metric_utils.h, only the implementation does.

Depending on otlp_metric_utils.h from a header file is problematic, because it brings a transitive dependency on protobuf generated code, which is not installed.

This fix allows an application to instantiate a OtlpHttpMetricExporter object, to configure the SDK to export metrics.

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

Merging #2179 (b7ca040) into main (500cf98) will decrease coverage by 0.02%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2179      +/-   ##
==========================================
- Coverage   87.52%   87.50%   -0.02%     
==========================================
  Files         169      169              
  Lines        4887     4887              
==========================================
- Hits         4277     4276       -1     
- Misses        610      611       +1     

see 1 file with indirect coverage changes