feat: add the UpDownCounter instrument

[mayurkale22]

Which problem is this PR solving?

• Fixes Implement UpDownCounter #1111

Short description of the changes

• Added new API endpoint and default SDK implementation.

• The Counter and SumObserver instruments are defined as monotonic (by default) and UpDown- variations of these two instruments are non-monotonic. So no need to have monotonic flag separately.

• Updated Prometheus export to export UpDownCounter as Gauge type.

• I will take care of examples and remaining documentation in subsequent PR.

[codecov]

Codecov Report

Merging #1120 into master will decrease coverage by 0.05%.
The diff coverage is 88.57%.

@@            Coverage Diff             @@
##           master    #1120      +/-   ##
==========================================
- Coverage   92.39%   92.33%   -0.06%     
==========================================
  Files         118      119       +1     
  Lines        3407     3433      +26     
  Branches      694      696       +2     
==========================================
+ Hits         3148     3170      +22     
- Misses        259      263       +4     

Impacted Files	Coverage Δ
packages/opentelemetry-api/src/metrics/Metric.ts	100.00% <ø> (ø)
...ackages/opentelemetry-api/src/metrics/NoopMeter.ts	69.76% <0.00%> (-3.41%)	⬇️
packages/opentelemetry-metrics/src/Metric.ts	100.00% <ø> (ø)
packages/opentelemetry-metrics/src/export/types.ts	100.00% <ø> (ø)
packages/opentelemetry-metrics/src/types.ts	100.00% <ø> (ø)
packages/opentelemetry-metrics/src/Meter.ts	93.65% <77.77%> (-2.65%)	⬇️
...pentelemetry-exporter-prometheus/src/prometheus.ts	91.39% <100.00%> (ø)
...kages/opentelemetry-metrics/src/BoundInstrument.ts	100.00% <100.00%> (ø)
...s/opentelemetry-metrics/src/UpDownCounterMetric.ts	100.00% <100.00%> (ø)
...ckages/opentelemetry-metrics/src/export/Batcher.ts	100.00% <100.00%> (ø)
... and 1 more

[obecny]

lgtm

[mwear]

LGTM