Add telemetry for various registry operations

We should add some telemetry events about various registry operations.

Initial ideas:
* Status codes and durations of ACR REST calls.
* Metrics around contention on module cache
* ...

Start by identifying the registry-operation entry points responsible for ACR REST calls and module-cache contention. Define the telemetry needed for REST status codes and durations, plus cache-contention metrics, then verify that the resulting events and metrics cover the listed registry operations.

Written by the indexing model from the issue text.