feat(otel): Centralize OTEL metric names and shared tag builders (#14117)

This cleans up a bunch of metric plumbing that had started to drift.

The main change is making `codex-otel` the canonical home for shared
metric definitions and metric tag helpers. I moved the `turn/thread`
metric names that were still duplicated into the OTEL metric registry,
added a shared `metrics::tags` module for common tag keys and session
tag construction, and updated `SessionTelemetry` to build its metadata
tags through that shared path.

On the codex-core side, TTFT/TTFM now use the shared metric-name
constants instead of local string definitions. I also switched the
obvious remaining turn/thread metric callsites over to the shared
constants, and added a small helper so TTFT/TTFM can attach an optional
sanitized client.name tag from TurnContext.

This should make follow-on telemetry work less ad hoc:
- one canonical place for metric names
- one canonical place for common metric tag keys/builders
- less duplication between `codex-core` and `codex-otel`

codex-rs/core/src/codex.rs

@@ -305,6 +305,7 @@ use crate::windows_sandbox::WindowsSandboxLevelExt;
 use codex_async_utils::OrCancelExt;
 use codex_otel::SessionTelemetry;
 use codex_otel::TelemetryAuthMode;
+use codex_otel::metrics::names::THREAD_STARTED_METRIC;
 use codex_protocol::config_types::CollaborationMode;
 use codex_protocol::config_types::Personality;
 use codex_protocol::config_types::ReasoningSummary as ReasoningSummaryConfig;
@@ -1412,7 +1413,7 @@ impl Session {
         };
         config.features.emit_metrics(&session_telemetry);
         session_telemetry.counter(
-            "codex.thread.started",
+            THREAD_STARTED_METRIC,
             1,
             &[(
                 "is_git",