feat(otlp-metric-exporters): Add User-Agent header to OTLP metric exporters

[JamieDanielson]

Which problem is this PR solving?

Updates #3291

Short description of the changes

• add user agent to otlp grpc metric exporter
• add user agent to otlp http/json metric exporter
• add user agent to otlp http/proto metric exporter
• add unit tests for the above changes

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

How Has This Been Tested?

• Unit tests

Checklist:

• Followed the style guidelines of this project
• Unit tests have been added
• Documentation has been updated

[codecov]

Codecov Report

Merging #3806 (6975248) into main (68eba71) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 6975248 differs from pull request most recent head cd88cd2. Consider uploading reports for the commit cd88cd2 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3806   +/-   ##
=======================================
  Coverage   92.94%   92.95%           
=======================================
  Files         295      295           
  Lines        9020     9026    +6     
  Branches     1841     1841           
=======================================
+ Hits         8384     8390    +6     
  Misses        636      636           

Impacted Files	Coverage Δ
...porter-metrics-otlp-grpc/src/OTLPMetricExporter.ts	93.54% <100.00%> (+0.44%)	⬆️
...-otlp-http/src/platform/node/OTLPMetricExporter.ts	100.00% <100.00%> (ø)
...orter-metrics-otlp-proto/src/OTLPMetricExporter.ts	100.00% <100.00%> (ø)

[pichlermarc]

Amazing, thank you 🙂