opentelemetry: initialize attemptInfo with fields set (#9262)

Fixes: #9097

Previously, if only tracing was enabled, the server-side span name would
not be correctly populated (falling back to "Recv.") because the method
name population logic was only present in the metrics handler's TagRPC
method.

This change fixes this by:
* Moving the common client RPC info initialization (including start time
and method name) to `getOrCreateClientRPCInfo`.
* Moving the common server RPC info initialization (including start
time, method name, and method filtering/validation logic) to
`getOrCreateServerRPCInfo`.
* Updating both client and server metrics and tracing handlers to use
these shared helper functions. This ensures that
the RPC info is correctly initialized by whichever handler executes
first.

RELEASE NOTES:
* opentelemetry: ensure method names are correctly set in trace spans
when metrics are disabled.