Add metrics abstractions and OpenTelemetry implementation plugin

[lmolkova]

In some cases (messaging, heavy and manually-written SDKs), it's useful to report metrics in addition to traces.
See Azure/azure-sdk#4389 for the design and champion scenarios.

Metrics for messaging SDKs have proven to be very useful in Java stress tests (and would be as useful for users).

As a sub-task, we should investigate if it's possible to reuse HTTP-metrics auto-collected for underlying HTTP clients (we might suppress them along with traces and need to find a way to suppress auto-collected traces, but not metrics).