chore: update metrics README

[mayurkale22]

Which problem is this PR solving?

• Update README

[draffensperger]

Would it make sense to add labels to these examples as well? (Maybe as a follow-up?)

[codecov-io]

Codecov Report

Merging #490 into master will increase coverage by 1.93%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #490      +/-   ##
==========================================
+ Coverage   90.35%   92.28%   +1.93%     
==========================================
  Files         149      152       +3     
  Lines        7534     7359     -175     
  Branches      663      649      -14     
==========================================
- Hits         6807     6791      -16     
+ Misses        727      568     -159

Impacted Files	Coverage Δ
...kages/opentelemetry-plugin-dns/test/utils/utils.ts	33.33% <0%> (-26.67%)	⬇️
...ages/opentelemetry-plugin-http/test/utils/utils.ts	33.33% <0%> (-26.67%)	⬇️
...es/opentelemetry-node/src/instrumentation/utils.ts	90.47% <0%> (-9.53%)	⬇️
...ges/opentelemetry-tracing/src/NoopSpanProcessor.ts	66.66% <0%> (-8.34%)	⬇️
...entelemetry-exporter-jaeger/test/transform.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/src/config.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/src/utility.ts	100% <0%> (ø)	⬆️
...core/src/context/propagation/NoopHttpTextFormat.ts	100% <0%> (ø)	⬆️
...telemetry-plugin-grpc/test/utils/assertionUtils.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-tracing/test/Span.test.ts	100% <0%> (ø)	⬆️
... and 37 more

[mayurkale22]

@open-telemetry/javascript-approvers Please review