[identity] Add gRPC request tracing span

Summary:
Way of improving logs. We have this pattern in Blob Service where each HTTP request has its unique ID. This way, we can trace logs for specific RPC call by filtering by this ID.
It doesn't decrease users' anonymity level - it doesn't introduce any fields that could let us trace user actions at level higher than a single RPC call.

Example of when such log IDs greatly improved debugging experience here

Test Plan:
Example screenshot here:

Reviewers: will

Reviewed By: will

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11985