Environment variable to set metrics producers

[gouthamve]

What are you trying to achieve?

I was trying to use the autoexport package in Golang SDK which configures the exporting and SDK through the environment variables in semantic conventions.

However, I needed to add the Prometheus metrics producer and there was no way to do it when using autoexport.

I'd like to propose a new environment variable OTEL_METRICS_PRODUCERS where users can pass comma separated producer values.

Additional context.

open-telemetry/opentelemetry-go-contrib#5281 has an implementation of this environment variable. cc @dashpole @pellared

I know there is a moratorium on new environment variables, but I'd like to atleast merge the above PR as EXPERIMENTAL to unblock my use-case.

[danielgblanco]

I believe this needs input from the TC as per this comment. I also added this to the Configuration SIG inbox, as they will be interested in this issue.

[jack-berg]

From a file configuration standpoint, metric producers are an SDK extension point. We support referencing custom implementations of SDK extension points via the component provider mechanism. If someone wants to open a PR to https://github.com/open-telemetry/opentelemetry-configuration to extend the schema with the metric producers, I'm supportive.

[jack-berg]

This is being added to file configuration in open-telemetry/opentelemetry-configuration#90

Discussed (briefly) in the 5/22 TC and we believe that file configuration should be sufficient and we should not add a new environment variable for this. I.e. under the moratorium this is considered non-essential.