feat: metrics capabilities (#8318)

Add metrics capabilities to Codex. The `README.md` is up to date.

This will not be merged with the metrics before this PR of course:
https://github.com/openai/codex/pull/8350

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

@@ -0,0 +1,68 @@
+use crate::harness::attributes_to_map;
+use crate::harness::build_metrics_with_defaults;
+use crate::harness::histogram_data;
+use crate::harness::latest_metrics;
+use codex_otel::metrics::Result;
+use pretty_assertions::assert_eq;
+use std::time::Duration;
+
+// Ensures duration recording maps to histogram output.
+#[test]
+fn record_duration_records_histogram() -> Result<()> {
+    let (metrics, exporter) = build_metrics_with_defaults(&[])?;
+
+    metrics.record_duration(
+        "codex.request_latency",
+        Duration::from_millis(15),
+        &[("route", "chat")],
+    )?;
+    metrics.shutdown()?;
+
+    let (bounds, bucket_counts, sum, count) =
+        histogram_data(&latest_metrics(&exporter), "codex.request_latency");
+    assert!(!bounds.is_empty());
+    assert_eq!(bucket_counts.iter().sum::<u64>(), 1);
+    assert_eq!(sum, 15.0);
+    assert_eq!(count, 1);
+
+    Ok(())
+}
+
+// Ensures time_result returns the closure output and records timing.
+#[test]
+fn timer_result_records_success() -> Result<()> {
+    let (metrics, exporter) = build_metrics_with_defaults(&[])?;
+
+    {
+        let timer = metrics.start_timer("codex.request_latency", &[("route", "chat")]);
+        assert!(timer.is_ok());
+    }
+
+    metrics.shutdown()?;
+
+    let resource_metrics = latest_metrics(&exporter);
+    let (bounds, bucket_counts, _sum, count) =
+        histogram_data(&resource_metrics, "codex.request_latency");
+    assert!(!bounds.is_empty());
+    assert_eq!(count, 1);
+    assert_eq!(bucket_counts.iter().sum::<u64>(), 1);
+    let attrs = attributes_to_map(
+        match crate::harness::find_metric(&resource_metrics, "codex.request_latency").and_then(
+            |metric| match metric.data() {
+                opentelemetry_sdk::metrics::data::AggregatedMetrics::F64(
+                    opentelemetry_sdk::metrics::data::MetricData::Histogram(histogram),
+                ) => histogram
+                    .data_points()
+                    .next()
+                    .map(opentelemetry_sdk::metrics::data::HistogramDataPoint::attributes),
+                _ => None,
+            },
+        ) {
+            Some(attrs) => attrs,
+            None => panic!("attributes missing"),
+        },
+    );
+    assert_eq!(attrs.get("route").map(String::as_str), Some("chat"));
+
+    Ok(())
+}