feat: Add prometheus exporter

[dyladan]

Which problem is this PR solving?

• SDK: Add Prometheus metrics exporter  #248 Add prometheus exporter

With #482 this now provides an end-to-end metrics story

Short description of the changes

Adds a metrics exporter for the prometheus backend. In its current state, the exporter maintains an internal state with the values of all exported metrics, which is then returned when an endpoint is called by Prometheus.

[codecov-io]

Codecov Report

Merging #483 into master will decrease coverage by 1.55%.
The diff coverage is 96.38%.

@@            Coverage Diff             @@
##           master     #483      +/-   ##
==========================================
- Coverage   92.03%   90.47%   -1.56%     
==========================================
  Files         138      144       +6     
  Lines        6264     7237     +973     
  Branches      527      640     +113     
==========================================
+ Hits         5765     6548     +783     
- Misses        499      689     +190

Impacted Files	Coverage Δ
...pentelemetry-exporter-prometheus/src/prometheus.ts	91.83% <91.83%> (ø)
...emetry-exporter-prometheus/test/prometheus.test.ts	98.54% <98.54%> (ø)
...pentelemetry-core/test/internal/validators.test.ts	50% <0%> (-50%)	⬇️
...elemetry-core/test/trace/spancontext-utils.test.ts	55.55% <0%> (-44.45%)	⬇️
...lemetry-core/test/trace/ProbabilitySampler.test.ts	56.52% <0%> (-43.48%)	⬇️
...s/opentelemetry-core/test/trace/NoopTracer.test.ts	60% <0%> (-40%)	⬇️
...s/opentelemetry-core/test/context/B3Format.test.ts	63.39% <0%> (-36.61%)	⬇️
...ges/opentelemetry-core/test/trace/NoopSpan.test.ts	63.63% <0%> (-36.37%)	⬇️
...s/opentelemetry-core/test/trace/tracestate.test.ts	65.06% <0%> (-34.94%)	⬇️
...kages/opentelemetry-metrics/test/mocks/Exporter.ts	66.66% <0%> (-33.34%)	⬇️
... and 45 more

[mayurkale22]

This is a great start, added a few comments in the first pass.

[mayurkale22]

Thank you @dyladan for this PR and for addressing my feedback! LGTM!

[draffensperger]

Is it possible this could cause duplicates? Should we write a warning in that case (could be a follow up task)?

[dyladan]

We could replace period with __DOT__ or similar

[mayurkale22]

I managed to run the end to end example using prometheus exporter, looks like everything is working fine. I tried creating and updating simple counter and gauge metrics every 2 secs.

/metrics endpoint:

On scraper: