Support OTEL_METRIC_EXPORT_INTERVAL and OTEL_METRIC_EXPORT_TIMEOUT

[pellared]

Feature Request

Support for OTEL_METRIC_EXPORT_INTERVAL and OTEL_METRIC_EXPORT_TIMEOUT environmental variables.

Reference: https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/sdk-environment-variables.md#periodic-exporting-metricreader

Is your feature request related to a problem?

See: open-telemetry/opentelemetry-dotnet-instrumentation#900

Describe the solution you'd like:

OTEL_METRIC_EXPORT_INTERVAL and OTEL_METRIC_EXPORT_TIMEOUT can be used to set PeriodicExportingMetricReaderOptions values if these were not programmatically set to non-default values.

Describe alternatives you've considered.

Implement it only on the https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation.

Additional Context

It can be handy for the users and it will also be reused in https://github.com/open-telemetry/opentelemetry-dotnet-instrumentation

It should be easy to implement it here: https://github.com/open-telemetry/opentelemetry-dotnet/blob/main/src/OpenTelemetry/Internal/PeriodicExportingMetricReaderHelper.cs