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/turn_timing.rs

@@ -1,6 +1,8 @@
 use std::time::Duration;
 use std::time::Instant;
 
+use codex_otel::metrics::names::TURN_TTFM_DURATION_METRIC;
+use codex_otel::metrics::names::TURN_TTFT_DURATION_METRIC;
 use codex_protocol::items::TurnItem;
 use codex_protocol::models::ResponseItem;
 use tokio::sync::Mutex;
@@ -9,9 +11,6 @@ use crate::ResponseEvent;
 use crate::codex::TurnContext;
 use crate::stream_events_utils::raw_assistant_output_text_from_item;
 
-const TURN_TTFT_DURATION_METRIC: &str = "codex.turn.ttft.duration_ms";
-const TURN_TTFM_DURATION_METRIC: &str = "codex.turn.ttfm.duration_ms";
-
 pub(crate) async fn record_turn_ttft_metric(turn_context: &TurnContext, event: &ResponseEvent) {
     let Some(duration) = turn_context
         .turn_timing_state