Add client-side exec-server RPC attempt metrics (#42883)

## What changed

- Record `exec_server_client_requests_total` for every client RPC call attempt,
  labeled by protocol method.
- Count attempts before local admission so rejected, timed-out, cancelled, and
  transport-failed calls are included, while notifications and responses are not.
- Export the counter to configured OpenTelemetry collectors while excluding it
  from the built-in Statsig metrics set.

## Testing

- Cover every RPC call entry point, successful responses, local and transport
  failures, timeouts, cancellation, notifications, and disabled metrics.
- Verify that the OTLP HTTP exporter retains the counter.

GitOrigin-RevId: d266e840d5871aa2c34bed8ce44f3345e2b4650f

codex-rs/exec-server/src/lib.rs

@@ -3,6 +3,7 @@ mod capability_discovery;
 mod capability_discovery_cache;
 mod client;
 mod client_api;
+mod client_telemetry;
 mod client_transport;
 mod connection;
 mod environment;