Remove the 'prometheus' feature and the deprecated opentelemetry-prometheus package

[jcjones]

The deprecation notice for opentelemetry-prometheus recommends opentelemetry_otlp as a replacement, which is already implemented in all our branches.

Once we are using OTLP in production, we can remove the deprecated package and unblock version conflicts among the various OpenTelemetry packages and this old Prometheus package.

[jcjones]

oh, ha, this is already implemented, or rather the feature is there.