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/harness/mod.rs

@@ -0,0 +1,81 @@
+use codex_otel::metrics::MetricsClient;
+use codex_otel::metrics::MetricsConfig;
+use codex_otel::metrics::Result;
+use opentelemetry::KeyValue;
+use opentelemetry_sdk::metrics::InMemoryMetricExporter;
+use opentelemetry_sdk::metrics::data::AggregatedMetrics;
+use opentelemetry_sdk::metrics::data::Metric;
+use opentelemetry_sdk::metrics::data::MetricData;
+use opentelemetry_sdk::metrics::data::ResourceMetrics;
+use std::collections::BTreeMap;
+
+pub(crate) fn build_metrics_with_defaults(
+    default_tags: &[(&str, &str)],
+) -> Result<(MetricsClient, InMemoryMetricExporter)> {
+    let exporter = InMemoryMetricExporter::default();
+    let mut config = MetricsConfig::in_memory(
+        "test",
+        "codex-cli",
+        env!("CARGO_PKG_VERSION"),
+        exporter.clone(),
+    );
+    for (key, value) in default_tags {
+        config = config.with_tag(*key, *value)?;
+    }
+    let metrics = MetricsClient::new(config)?;
+    Ok((metrics, exporter))
+}
+
+pub(crate) fn latest_metrics(exporter: &InMemoryMetricExporter) -> ResourceMetrics {
+    let Ok(metrics) = exporter.get_finished_metrics() else {
+        panic!("finished metrics error");
+    };
+    let Some(metrics) = metrics.into_iter().last() else {
+        panic!("metrics export missing");
+    };
+    metrics
+}
+
+pub(crate) fn find_metric<'a>(
+    resource_metrics: &'a ResourceMetrics,
+    name: &str,
+) -> Option<&'a Metric> {
+    for scope_metrics in resource_metrics.scope_metrics() {
+        for metric in scope_metrics.metrics() {
+            if metric.name() == name {
+                return Some(metric);
+            }
+        }
+    }
+    None
+}
+
+pub(crate) fn attributes_to_map<'a>(
+    attributes: impl Iterator<Item = &'a KeyValue>,
+) -> BTreeMap<String, String> {
+    attributes
+        .map(|kv| (kv.key.as_str().to_string(), kv.value.as_str().to_string()))
+        .collect()
+}
+
+pub(crate) fn histogram_data(
+    resource_metrics: &ResourceMetrics,
+    name: &str,
+) -> (Vec<f64>, Vec<u64>, f64, u64) {
+    let metric =
+        find_metric(resource_metrics, name).unwrap_or_else(|| panic!("metric {name} missing"));
+    match metric.data() {
+        AggregatedMetrics::F64(data) => match data {
+            MetricData::Histogram(histogram) => {
+                let points: Vec<_> = histogram.data_points().collect();
+                assert_eq!(points.len(), 1);
+                let point = points[0];
+                let bounds = point.bounds().collect();
+                let bucket_counts = point.bucket_counts().collect();
+                (bounds, bucket_counts, point.sum(), point.count())
+            }
+            _ => panic!("unexpected histogram aggregation"),
+        },
+        _ => panic!("unexpected metric data type"),
+    }
+}