Configure OTLP metrics exporter with environment variables

[legendecas]

https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/sdk_exporters/otlp.md

OTLP exporter spec defines that the OTLP exporter should be automatically configured with a PeriodicMetricReader when using OTEL_METRICS_EXPORTER. Their aggregation and temporality preferences should also be configured with OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE and OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION.

[dyladan]

I saw you added this to the metrics GA milestone. Do you believe this is blocking the metrics SDK?

[legendecas]

No, I believe it's not blocking but rather good to have.

[pichlermarc]

Just went through the code and saw that the following env-vars from this spec are also not implemented right now:

• OTEL_EXPORTER_OTLP_METRICS_COMPRESSION (currently the trace setting is used instead)
• OTEL_EXPORTER_OTLP_METRICS_INSECURE (currently the trace setting is used instead)
• OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE (currently the trace setting is used instead)
• OTEL_EXPORTER_OTLP_METRICS_CLIENT_KEY (currently the trace setting is used instead)
• OTEL_EXPORTER_OTLP_METRICS_CLIENT_CERTIFICATE (currently the trace setting is used instead)

[github-actions]

This issue is stale because it has been open 60 days with no activity. Remove stale label or comment or this will be closed in 14 days.

[pichlermarc]

Not stale as we'd need to implement OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION after #3506 lands. 🙂
(we also need to address the comment I made earlier)

[pichlermarc]

When this is done, we'll also need to update the spec compliance matrix with the newly supported env vars.

[pichlermarc]

Just went through the code and saw that the following env-vars from this spec are also not implemented right now:

* `OTEL_EXPORTER_OTLP_METRICS_COMPRESSION` (currently the trace setting is used instead)

* `OTEL_EXPORTER_OTLP_METRICS_INSECURE` (currently the trace setting is used instead)

* `OTEL_EXPORTER_OTLP_METRICS_CERTIFICATE` (currently the trace setting is used instead)

* `OTEL_EXPORTER_OTLP_METRICS_CLIENT_KEY` (currently the trace setting is used instead)

* `OTEL_EXPORTER_OTLP_METRICS_CLIENT_CERTIFICATE` (currently the trace setting is used instead)

This should work now as of #4971 🙂
OTEL_METRICS_EXPORTER auto-pairing will be implemented in #4551

I think we're only missing OTEL_EXPORTER_OTLP_METRICS_DEFAULT_HISTOGRAM_AGGREGATION now, temporality preference is implemented in #3305