Open Telemetry stats sink support

[ohadvano]

I can see that stats sinks currently do not have an open telemetry extension. Is there currently WIP to support it or are there any known issues with supporting OTEL metrics sink? If not, will appreciate marking this issue as help wanted.

Implementation-wise, it seems like "Metrics service" extension is doing most of the work, but it's using Prometheus metrics. Maybe it's possibly that most of the code can be extracted to common folder and have "Metrics service" and "otel_grpc" extension share the common code?

cc @snowp