[codex] add latency tracing spans (#27710)

## Why

We have some large gaps in our thread start, resume, and pre-sampling
traces that make it hard to tell where latency is coming from.

## What Changed

- Added coarse spans around thread start/resume, turn context
construction, rollout reconstruction, skill/plugin loading, and tool
preparation.
- Added a breakdown of discoverable-tool preparation across connector
loading, plugin discovery, and local plugin details.

## Testing

- `cargo check -p codex-app-server -p codex-core -p codex-core-skills -p
codex-core-plugins`
- Built the app-server locally and exercised thread start, first turn,
follow-up turn, server restart, thread resume, and a resumed turn.

codex-rs/core/src/connectors.rs

@@ -18,6 +18,7 @@ use codex_tools::DiscoverableTool;
 use rmcp::model::ToolAnnotations;
 use serde::Deserialize;
 use tokio_util::sync::CancellationToken;
+use tracing::instrument;
 use tracing::warn;
 
 use crate::config::Config;
@@ -460,6 +461,7 @@ fn tool_suggest_connector_ids(
     connector_ids
 }
 
+#[instrument(level = "trace", skip_all)]
 async fn cached_directory_connectors_for_tool_suggest_with_auth(
     config: &Config,
     auth: Option<&CodexAuth>,

codex-rs/core/src/plugins/discoverable.rs

@@ -5,7 +5,9 @@ use codex_core_plugins::ToolSuggestPluginDiscoveryInput;
 use codex_login::CodexAuth;
 use codex_tools::DiscoverablePluginInfo;
 use std::collections::HashSet;
+use tracing::instrument;
 
+#[instrument(level = "trace", skip_all)]
 pub(crate) async fn list_tool_suggest_discoverable_plugins(
     config: &Config,
     plugins_manager: &PluginsManager,

codex-rs/core/src/session/mod.rs

@@ -1308,6 +1308,14 @@ impl Session {
         }
     }
 
+    #[instrument(
+        level = "trace",
+        skip_all,
+        fields(
+            thread_id = %self.thread_id(),
+            rollout_item_count = rollout_items.len()
+        )
+    )]
     async fn apply_rollout_reconstruction(
         &self,
         turn_context: &TurnContext,

codex-rs/core/src/session/turn_context.rs

@@ -20,6 +20,7 @@ use codex_sandboxing::policy_transforms::effective_network_sandbox_policy;
 use codex_utils_path_uri::PathUri;
 use std::sync::atomic::AtomicBool;
 use std::sync::atomic::Ordering;
+use tracing::instrument;
 
 #[derive(Clone, Debug)]
 pub(crate) struct TurnSkillsContext {
@@ -724,6 +725,7 @@ impl Session {
         .await
     }
 
+    #[instrument(name = "turn_context.build", level = "trace", skip_all)]
     async fn new_turn_context_from_configuration(
         &self,
         sub_id: String,

codex-rs/core/src/thread_manager.rs

@@ -78,6 +78,7 @@ use std::sync::atomic::Ordering;
 use std::time::Duration;
 use tokio::sync::RwLock;
 use tokio::sync::broadcast;
+use tracing::instrument;
 use tracing::warn;
 
 const THREAD_CREATED_CHANNEL_CAPACITY: usize = 1024;
@@ -702,6 +703,7 @@ impl ThreadManager {
         .await
     }
 
+    #[instrument(level = "trace", skip_all)]
     pub async fn resume_thread_with_history(
         &self,
         config: Config,