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/otel/tests/suite/otlp_http_loopback.rs

@@ -1,3 +1,4 @@
+use codex_otel::EXEC_SERVER_CLIENT_REQUEST_COUNT_METRIC;
 use codex_otel::MetricsClient;
 use codex_otel::MetricsConfig;
 use codex_otel::OtelExporter;
@@ -187,6 +188,12 @@ fn otlp_http_exporter_sends_metrics_to_collector() -> Result<()> {
 
     metrics.counter("codex.turns", /*inc*/ 1, &[("source", "test")])?;
     metrics.counter("codex.api_request", /*inc*/ 1, &[("status", "200")])?;
+    metrics.counter_with_description(
+        EXEC_SERVER_CLIENT_REQUEST_COUNT_METRIC,
+        "Client-side exec-server RPC attempts.",
+        /*inc*/ 1,
+        &[("method", "fs/readFile")],
+    )?;
     metrics.record_duration(
         "codex.api_request.duration_ms",
         Duration::from_millis(100),
@@ -259,6 +266,10 @@ fn otlp_http_exporter_sends_metrics_to_collector() -> Result<()> {
         "expected API-request counter not found; body prefix: {}",
         &body.chars().take(2000).collect::<String>()
     );
+    assert!(
+        body.contains(EXEC_SERVER_CLIENT_REQUEST_COUNT_METRIC),
+        "custom OTLP must retain the exec-server client counter excluded from built-in Statsig"
+    );
     assert!(
         body.contains("\"codex.api_request.duration_ms\""),
         "expected API-request duration not found; body prefix: {}",