feat: export unit block when unit is set in metric descriptor

[weyert]

Which problem is this PR solving?

Updated the PrometheusSerializer so that it exports the # UNIT block of the Prometheus format when the unit field of a metric descriptor is set

Fixes #3015 (issue)

Short description of the changes

Added support for exporting the # UNIT metadata of a metric

Type of change

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

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• Ensured the # UNIT-line is exported for a metric that has unit set

Checklist:

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

[codecov]

Codecov Report

Merging #3041 (355fc6a) into main (df58fac) will decrease coverage by 0.00%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #3041      +/-   ##
==========================================
- Coverage   93.07%   93.06%   -0.01%     
==========================================
  Files         195      162      -33     
  Lines        6384     4428    -1956     
  Branches     1347      886     -461     
==========================================
- Hits         5942     4121    -1821     
+ Misses        442      307     -135     

Impacted Files	Coverage Δ
...ry-exporter-prometheus/src/PrometheusSerializer.ts
...ental/packages/otlp-grpc-exporter-base/src/util.ts
...ges/opentelemetry-instrumentation-http/src/http.ts
...orter-metrics-otlp-proto/src/OTLPMetricExporter.ts
...imental/packages/opentelemetry-sdk-node/src/sdk.ts
.../packages/otlp-transformer/src/metrics/internal.ts
...ry-instrumentation-grpc/src/grpc-js/serverUtils.ts
...es/opentelemetry-instrumentation-grpc/src/utils.ts
...y-instrumentation-http/src/enums/AttributeNames.ts
...tal/packages/otlp-transformer/src/metrics/index.ts
... and 23 more

[legendecas]

Thank you for your work on this! Overall LGTM % a suggestion on tests.

[dyladan]

@weyert any update on the comments and conflict?

[weyert]

Picking this up, been busy switching to latest otel-js at work. Going to have a look at this again. Should have it stashed away somewhere

[dyladan]

Friendly ping :)

[weyert]

@open-telemetry/javascript-maintainers I have updated the PR based on the feedback :)