Support the OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE environment variable

[pichlermarc]

Problem Statement

Environment variables like OTEL_EXPORTER_OTLP_ENDPOINT and OTEL_EXPORTER_OTLP_HEADERS are already supported. However, currently, the temporality preference cannot be configured via environment variables.

Proposed Solution

Support the OTEL_EXPORTER_OTLP_METRICS_TEMPORALITY_PREFERENCE environment variable, so that it can be configured in the same manner as the endpoint and headers. This might only be feasible to implement once #3260 is merged, as it then becomes easier to consider this a part of the exporter configuration.

Additional Context

Releveant specification

[MrAlias]

If a language provides a mechanism to automatically configure a MetricReader to pair with the associated Exporter (e.g., using the OTEL_METRICS_EXPORTER environment variable), then by default:

This project currently does not support that, nor are their plans to support it due to the inability to limit dependencies. It may be something the autoexport package could include (see open-telemetry/opentelemetry-go-contrib#2753), but that is a ways from doing so.

There is also talk that this functionality extends beyond the scope of the exporter as well: open-telemetry/opentelemetry-specification#2810 and open-telemetry/opentelemetry-specification#2854. These will all need to be resolved before it can be included here or in the contrib repository.