Document latest Metrics SDK

[legendecas]

We need to update the standalone documents of Metrics SDK to help people understand the metrics spec changes and how JavaScript SDK implements them.

• https://github.com/open-telemetry/opentelemetry-js/blob/main/doc/processor-api.md needs to be replaced with Views API
• https://github.com/open-telemetry/opentelemetry-js/blob/main/doc/exporter-guide.md#metrics needs to be replaced with MetricReader and MetricExporter
  • Details about the difference between two AggregationTemporality.
  • Examples for both pull/push models of exporting.
• Examples for all kinds of instruments and common practice.
  • Significant changes in histogram and boundaries.
  • Significant changes in async instruments Consolidate Observer and Counter example #868.

[dyladan]

I believe all docs should be moved to the website. We can iterate on them here and move them to the website when we're happy with them?

[github-actions]

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[cartermp]

This was discussed a bit at the maintainer's meeting last week. On the docs side, we have outlines that can be followed now!

Getting started: open-telemetry/opentelemetry.io#1746 (comment)
Manual instrumentation: open-telemetry/opentelemetry.io#1746 (comment)

As mentioned in the call, if y'all can contribute a code sample that handles initialization of metrics in the most correct way, then we (docs SIG) can probably cover the rest iteratively.

I also think that JS getting started is unique, since there's really two major SDKs that people use, whereas other languages just have one way to get started. I'd recommend that for now, we focus on the Manual Instrumentation doc and then figure out the right way to structure the JS getting started docs orthogonally.

[dyladan]

Sorry to leave you hanging for so long I promise I haven't forgotten this. I'll make sure to get something tomorrow even if it isn't completely polished so you have something to work with.

[cartermp]

Sounds good!