[METRICS SDK] Fix OTLP gRPC Metrics env variables

[lalitb]

Fixes # (issue)

Changes

Please provide a brief description of the changes here.

otlp gRPC Metrics exporter is reusing the env variable processing from otlp trace exporter, which is making it use env variables containing _TRACE_ instead of _METRICS_. This is fixed now.

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[bot]]

Codecov Report

Merging #1543 (aa07289) into main (8b61c99) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #1543   +/-   ##
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  Coverage   83.91%   83.91%           
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  Files         156      156           
  Lines        4908     4908           
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  Hits         4118     4118           
  Misses        790      790