Add process diagnostics snapshots (#37434)

## What changed

- Add a `codex-diagnostics` crate that snapshots the process ID, available
  resident-memory measurements, and registered process-wide gauges.
- Provide guards that update gauges for the lifetime of measured objects.
- Track live `CodexThread` instances with the `core.threads.live` gauge.

## Testing

- Add unit coverage for gauge registration, guard lifetimes, process memory
  snapshots, and live-thread reporting.

GitOrigin-RevId: 3236b086bd4ebe31ed4768ab87a5fa288b0891b0

codex-rs/core/src/codex_thread.rs

@@ -9,6 +9,8 @@ use crate::session::session::Session;
 use crate::user_message_admission::PendingUserMessageAdmissionState;
 use crate::user_message_admission::UserMessageAdmission;
 use crate::user_message_admission::UserMessageAdmissionError;
+use codex_diagnostics::Gauge;
+use codex_diagnostics::GaugeGuard;
 use codex_exec_server::SelectedCapabilityRootsStatus;
 use codex_extension_api::ThreadIdleCause;
 use codex_features::Feature;
@@ -65,6 +67,8 @@ use tokio_util::sync::CancellationToken;
 
 use codex_rollout::state_db::StateDbHandle;
 
+static LIVE_THREADS: Gauge = Gauge::new("core.threads.live");
+
 #[derive(Clone, Debug)]
 pub struct ThreadConfigSnapshot {
     pub model: String,
@@ -197,6 +201,7 @@ pub struct CodexThread {
     session_configured: SessionConfiguredEvent,
     rollout_path: Option<PathBuf>,
     out_of_band_elicitations: Mutex<OutOfBandElicitations>,
+    _diagnostics_guard: GaugeGuard,
 }
 
 #[derive(Default)]
@@ -230,6 +235,7 @@ impl CodexThread {
             session_configured,
             rollout_path,
             out_of_band_elicitations: Mutex::new(OutOfBandElicitations::default()),
+            _diagnostics_guard: LIVE_THREADS.track(),
         }
     }
 

codex-rs/core/src/thread_manager_tests.rs

@@ -800,6 +800,12 @@ async fn start_thread_keeps_internal_threads_hidden_from_normal_lookups() {
 
     assert_eq!(manager.list_thread_ids().await, Vec::new());
     assert!(manager.get_thread(thread.thread_id).await.is_err());
+    assert!(
+        codex_diagnostics::snapshot()
+            .gauges
+            .iter()
+            .any(|gauge| gauge.name == "core.threads.live" && gauge.value > 0)
+    );
 
     let report = manager
         .shutdown_all_threads_bounded(Duration::from_secs(10))