Ensure that OpenTelemetry's OTEL_ environment variables are no-op

[AlekSi]

What should be done?

We already have some code that explicitly passes all parameters to prevent the OTel SDK from reading default values from environment variables. We should check it and add the missing parameters.

Maybe we should os.Unsetenv all OTEL_ variables from the environment in some init function?

https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/

We should also document that configuring OpenTelemetry via OTEL_ variables is not supported.

Where?

TBD

Definition of Done

TBD

• handler updated;
• unit tests added/updated;
• integration/compatibility tests added/updated;
• spot refactorings done;
• user documentation updated (or an issue to create documentation created);
• something else?