[sdk-node] automatically configure metrics exporter based on enviornment variables

[pichlermarc]

Currently, we don't auto-configure a MetricReader/exporter combination when using the @opentelemetry/sdk-node package.
Goal of this issue is to implement exporter selection for metrics based on this specification:

• https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#exporter-selection

For this issue to be considered done we need to implement the following behavior:

If no metric reader is configured by the user:

• use OTEL_METRICS_EXPORTER environment variable to determine an exporter and add it to the MeterProvider that's created by NodeSDK
  • pair it up with a PeriodicExportingMetricReader if it is a PushMetricExporter
  • see https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/configuration/sdk-environment-variables.md#exporter-selection
• use the OTEL_EXPORTER_OTLP_METRICS_PROTOCOL to determine the OTLP exporter to use (http/json, http/protobuf, grpc)
  • see https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/protocol/exporter.md#specify-protocol
• use the OTEL_EXPORTER_OTLP_PROTOCOL env var as a fallback to the above to determine the OTLP exporter to use (http/json, http/protobuf, grpc)
• fallback to using http/protobuf if none of the two protocol env vars are set

[Prashansa-K]

@pichlermarc I would like to work on this. I have used this package in the past.

[pichlermarc]

@Prashansa-K thanks for picking this up. It's yours 🙂

[Prashansa-K]

@pichlermarc
I have some doubts here.

1. In first action item from the description mentions "use OTEL_METRICS_EXPORTER environment variable to determine an exporter and add it to the MeterProvider that's created by NodeSDK",
   However, as I see the code, addMetricReader() method is deprecated and would be removed in v2. It is suggests to use the MeterProvider constructor here.
   So, should we keep creating new MeterProvider objects here based on the exporters provided via env variable?

2. I am assuming that we can provide >1 metric exporters via env configurations, just like trace exporters. Is that correct? I couldn't find a clarification for this in the specification document.

[pichlermarc]

So, should we keep creating new MeterProvider objects here based on the exporters provided via env variable?

If the env var is set, only create a single MeterProvider and add all MetricReaders to that MeterProvider via the constructor option (it's an array). That means that creation of the exporters with auto-paired MetricReaders has to happen before instantiating a MeterProvider.

I am assuming that we can provide >1 metric exporters via env configurations, just like trace exporters. Is that correct? I couldn't find a clarification for this in the specification document.

That's correct. 🙂 The specification reads:

The implementation MAY accept a comma-separated list to enable setting multiple exporters.

so that's applicable to all exporter selection env vars.