feat(exporter-metrics-otlp-proto): add esm build

[pichlermarc]

Which problem is this PR solving?

Looks like we didn't build esm and esnext for @opentelemetry/exporter-metrics-otlp-proto. This PR adds a build for it.

Fixes #4097

Type of change

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

[codecov]

Codecov Report

Merging #4099 (0e13d67) into main (ed27a37) will not change coverage.
The diff coverage is n/a.

❗ Current head 0e13d67 differs from pull request most recent head 938f6d2. Consider uploading reports for the commit 938f6d2 to get more accurate results

@@           Coverage Diff           @@
##             main    #4099   +/-   ##
=======================================
  Coverage   92.63%   92.63%           
=======================================
  Files         294      294           
  Lines        8264     8264           
  Branches     1714     1714           
=======================================
  Hits         7655     7655           
  Misses        609      609           

[dyladan]

This is fine but we should reconsider our esm publishing strategy anyway since these esm versions aren't used by node esm