feat: Metrics SDK - aggregator, batcher, controller

[mayurkale22]

Short description of the changes

• Implements several of the specs from : open-telemetry/opentelemetry-specification#347
• Uses alot of elements from : open-telemetry/opentelemetry-go#265.
• See this diagram for an overview of how the pieces work together.
• This adds basic aggregators (CounterSumAggregator, GaugeAggregator and MeasureExactAggregator), batcher (UngroupedBatcher) and controller (PushController).

TODO:

• Update all README files
• Update examples
• JSDoc comments
• Measure record (https://github.com/open-telemetry/opentelemetry-js/blob/master/packages/opentelemetry-metrics/src/Meter.ts#L69) - will do it in follow-up PR.
  Original PR: skeleton(metrics): createMeasure #563

Note:

• Submitting PR to get early feedback on API, Usage etc.
• I am more than happy to split this work in smaller PRs. For now wanted to give a complete sense of implementation and to see how all things fit toghther.

[codecov-io]

Codecov Report

Merging #738 into master will decrease coverage by 12.12%.
The diff coverage is 77.77%.

@@             Coverage Diff             @@
##           master     #738       +/-   ##
===========================================
- Coverage   92.55%   80.42%   -12.13%     
===========================================
  Files         240       99      -141     
  Lines       10819     3776     -7043     
  Branches      990      333      -657     
===========================================
- Hits        10013     3037     -6976     
+ Misses        806      739       -67

Impacted Files	Coverage Δ
...metry-core/src/trace/instrumentation/BasePlugin.ts	86.84% <ø> (ø)	⬆️
...opentelemetry-node/test/NodeTracerProvider.test.ts	100% <ø> (ø)
...ackages/opentelemetry-api/src/metrics/NoopMeter.ts	81.13% <ø> (ø)	⬆️
packages/opentelemetry-api/src/metrics/Metric.ts	100% <ø> (ø)	⬆️
...pentelemetry-core/test/internal/validators.test.ts	0% <0%> (-100%)	⬇️
...ages/opentelemetry-core/src/internal/validators.ts	100% <100%> (ø)	⬆️
...try-node/test/instrumentation/PluginLoader.test.ts	100% <100%> (ø)	⬆️
...telemetry-node/src/instrumentation/PluginLoader.ts	92.53% <50%> (-1.12%)	⬇️
...entelemetry-core/test/common/ConsoleLogger.test.ts	0% <0%> (-100%)	⬇️
...try-core/test/trace/fixtures/test-package/index.js	0% <0%> (-100%)	⬇️
... and 195 more

[dyladan]

This is really an awesome start, thanks for this. It looks like a pretty big improvement on the old version.

[dyladan]

Shouldn't the counter always use the sum aggregator?

[dyladan]

I know we talked about this in the SIG meeting a couple weeks ago, but I don't remember the resolution.

[mayurkale22]

As discussed offline, hardcoding will certainly simplify the things and will be less buggy (and easy to read). Also, you're right (99.9%) that counter should always have CounterSumAggregator. But I haven't seen any hardcoded stuff in specs or Go/Python implementations. Aggregators are always coming from Batcher(aggregatorFor - function). I was thinking to keep it as it is and open an issue to consider doing it in the future, once specs is completely flushed out.

[mayurkale22]

@open-telemetry/javascript-approvers We need reviews for this one, please take a look when you get a chance.

[dyladan]

Overall this looks good, just have a few unresolved questions from before.

[dyladan]

I know we talked about this in the SIG meeting a couple weeks ago, but I don't remember the resolution.

[dyladan]

Think I'm happy with this as a good place we can iterate off of. Thanks for doing this :)

[vmarchaud]

lgtm

[dyladan]

I think this is ready to merge if you're happy with it @mayurkale22

[obecny]

guys changes here caused the example for Prometheus is not working anymore

[mayurkale22]

Thanks for reporting this, will try to fix it today.