OTEL_METRICS_EXPORTER implementation

[TBBle]

Is your feature request related to a problem?

You cannot create a metrics exporter pipeline with environment variables.

Describe the solution you'd like

The OpenTelemetry-standardised OTEL_METRICS_EXPORTER should be implemented for its "known values".

It'd be nice if as well as the "known values", a value was chosen for the "In-memory Metrics Exporter". I haven't looked around at other SDKs to see if they do this, but in this SDK, that's implemented as an InMemoryMetricReader instead, so this might be more complex, and require some extra variables, similar to the OTEL_PYTHON_METER_PROVIDER environment variable, or a simple InMemoryMetricExporter that wraps the InMemoryMetricReader. (Alternatively, convert the InMemoryMetricReader to be a spec-compliant "In-memory Metrics Exporter" that works with PeriodicExportingMetricReader, I guess, or pull the existing one out of the test suite and clean it up a bit.)

Describe alternatives you've considered

We have to do all our metrics setup in the code, which requires plumbing our configuration through from the environment (or test suite hosting system).

Additional context

Structurally, I assume it would be added in the same place as the OTEL_TRACES_EXPORTER and OTEL_LOGS_EXPORTER are handled.

Okay, I had a quick look around, and possibly only the Java SDK has an InMemoryMetricExporter implementation, unless the spelling is different and fooled GitHub search. A few things implement OTEL_METRICS_EXPORTER for otlp and none though, e.g. the .NET and PHP SDKs.

[lzchen]

Should be address by: #2705

[TBBle]

#2705 has delivered the support for processing OTEL_METRICS_EXPORTER, but there is still another PR to go to add opentelemetry_metrics_exporter entry points for the SDK-provided exporters. Particularly since ConsoleMetricsExporter is still broken (#2716), and we don't have an InMemoryMetricsExporter.

[srikanthccv]

There won't be another InMemoryMetricsExporter component. See the #2539 for prior context. The ConsoleMetricsExporter bug is addressed here #2722 .

[srikanthccv]

Configuration is updated #1705, entry points are added #2748, and default grpc exporter set for opentelemetry-instrument open-telemetry/opentelemetry-python-contrib#1127. Closing this since all of relevant work is done. Feel free to re-open if something is missing.