Export turn cost as an OTEL metric (#40488)

## What changed

- Emit `codex.turn.cost_microusd` as a counter with turn, conversation,
  interruption, speed, and reasoning-effort attributes. Convert the estimated
  USD string to microdollars, rounding to the nearest microdollar and skipping
  values that cannot be represented safely.
- Start turn-cost collection when an OTLP metrics exporter is configured, even
  when the OTEL log exporter is disabled.

## Testing

- Verify cost conversion, rounding, and metric attributes with an in-memory
  metrics snapshot.
- Verify that a metrics-only OTLP configuration starts the turn-cost worker.

GitOrigin-RevId: e60587079f097f10cc1b6d1bf889d3f7a0dc3d5f

codex-rs/otel/tests/suite/snapshot.rs

@@ -155,3 +155,68 @@ fn manager_snapshot_metrics_collects_without_shutdown() -> Result<()> {
 
     Ok(())
 }
+
+#[test]
+fn manager_turn_cost_records_microusd_metric() -> Result<()> {
+    let exporter = InMemoryMetricExporter::default();
+    let config = MetricsConfig::in_memory("test", "codex-cli", env!("CARGO_PKG_VERSION"), exporter)
+        .with_runtime_reader();
+    let metrics = MetricsClient::new(config)?;
+    let thread_id = ThreadId::new();
+    let conversation_id = thread_id.to_string();
+    let manager = SessionTelemetry::new(
+        thread_id,
+        "gpt-5.6",
+        "gpt-5.6",
+        /*account_id*/ None,
+        /*account_email*/ None,
+        Some(TelemetryAuthMode::ApiKey),
+        "test_originator".to_string(),
+        /*log_user_prompts*/ false,
+        "tty".to_string(),
+        SessionSource::Cli,
+    )
+    .with_metrics(metrics);
+
+    manager.record_turn_cost(
+        "turn-123",
+        "0.0001245",
+        /*interrupted*/ false,
+        Some("fast"),
+        Some("high"),
+    );
+
+    let snapshot = manager.snapshot_metrics()?;
+    let metric = find_metric(&snapshot, "codex.turn.cost_microusd")
+        .expect("turn-cost microdollar metric missing");
+    let point = match metric.data() {
+        AggregatedMetrics::U64(MetricData::Sum(sum)) => {
+            sum.data_points().next().expect("turn-cost data point")
+        }
+        _ => panic!("unexpected turn-cost metric data type"),
+    };
+    assert_eq!(point.value(), 125);
+    assert_eq!(
+        attributes_to_map(point.attributes()),
+        BTreeMap::from([
+            (
+                "app.version".to_string(),
+                env!("CARGO_PKG_VERSION").to_string(),
+            ),
+            (
+                "auth_mode".to_string(),
+                TelemetryAuthMode::ApiKey.to_string(),
+            ),
+            ("conversation.id".to_string(), conversation_id),
+            ("model".to_string(), "gpt-5.6".to_string()),
+            ("originator".to_string(), "test_originator".to_string()),
+            ("reasoning_effort".to_string(), "high".to_string()),
+            ("session_source".to_string(), "cli".to_string()),
+            ("speed".to_string(), "fast".to_string()),
+            ("turn.id".to_string(), "turn-123".to_string()),
+            ("turn.interrupted".to_string(), "false".to_string()),
+        ])
+    );
+
+    Ok(())
+}