Remove label set from metrics API

[mayurkale22]

Which problem is this PR solving?

• Fixes Remove label set from metrics API #865

Short description of the changes

• Remove LabelSet according to OTEP 90:
  https://github.com/open-telemetry/oteps/blob/master/text/0090-remove-labelset-from-metrics-api.md

[codecov-io]

Codecov Report

Merging #915 into master will increase coverage by 16.32%.
The diff coverage is 93.06%.

@@             Coverage Diff             @@
##           master     #915       +/-   ##
===========================================
+ Coverage   78.62%   94.95%   +16.32%     
===========================================
  Files          73      247      +174     
  Lines        1736    11153     +9417     
  Branches      300     1067      +767     
===========================================
+ Hits         1365    10590     +9225     
- Misses        371      563      +192     

Impacted Files	Coverage Δ
packages/opentelemetry-api/src/metrics/Metric.ts	100.00% <ø> (ø)
packages/opentelemetry-metrics/src/Meter.ts	96.29% <ø> (ø)
packages/opentelemetry-metrics/src/export/types.ts	100.00% <ø> (ø)
...ackages/opentelemetry-api/src/metrics/NoopMeter.ts	72.72% <28.57%> (-1.75%)	⬇️
packages/opentelemetry-metrics/src/Metric.ts	96.66% <88.23%> (ø)
...y-api/test/noop-implementations/noop-meter.test.ts	100.00% <100.00%> (ø)
...pentelemetry-exporter-prometheus/src/prometheus.ts	92.00% <100.00%> (ø)
...emetry-exporter-prometheus/test/prometheus.test.ts	98.70% <100.00%> (ø)
...kages/opentelemetry-metrics/src/BoundInstrument.ts	92.85% <100.00%> (ø)
...ckages/opentelemetry-metrics/src/ObserverResult.ts	100.00% <100.00%> (ø)
... and 190 more