chore: update prometheus exporter readme with usage and links

[mayurkale22]

Which problem is this PR solving?

• Add USAGE instructions.

/cc @dyladan

[codecov-io]

Codecov Report

Merging #521 into master will increase coverage by 43.27%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #521       +/-   ##
===========================================
+ Coverage    49.2%   92.48%   +43.27%     
===========================================
  Files          47      146       +99     
  Lines        1392     7213     +5821     
  Branches      143      629      +486     
===========================================
+ Hits          685     6671     +5986     
+ Misses        707      542      -165

Impacted Files	Coverage Δ
.../opentelemetry-core/src/trace/spancontext-utils.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-web/src/WebTracer.ts	100% <0%> (ø)	⬆️
...opentelemetry-core/src/trace/globaltracer-utils.ts	100% <0%> (ø)	⬆️
.../opentelemetry-core/test/metrics/NoopMeter.test.ts	97.22% <0%> (ø)
...emetry-metrics/src/export/ConsoleMetricExporter.ts	93.75% <0%> (ø)
...entelemetry-plugin-dns/src/enums/AttributeNames.ts	100% <0%> (ø)
...ry-plugin-dns/test/functionals/dns-disable.test.ts	100% <0%> (ø)
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)
...opentelemetry-base/test/resources/resource.test.ts	100% <0%> (ø)
packages/opentelemetry-tracing/src/config.ts	100% <0%> (ø)
... and 111 more

[dyladan]

LGTM. Maybe add a step to start prometheus with a docker command?

[mayurkale22]

LGTM. Maybe add a step to start prometheus with a docker command?

https://opencensus.io/codelabs/prometheus/#0 <= link is already included for setup.