Feature: Emit group stats information

[lyuboxa]

Feature description

It will be useful to have the group stats like lag, hw and per partition offset emitted in the log to keep track of progress.

[lovromazgon]

By default emitting it every 20-30 seconds would be enough at first, maybe make it configurable. In the future we could roll up the connector metrics into the Conduit metrics exposed through the /metrics endpoint by exposing a "metrics connector utility service" (similar to the schema service we are introducing in v0.11.0).