Refine additional-context byte histogram buckets (#58939)

## Why

The previous histogram placed post-truncation values just over 4,000 bytes in a bucket ending at 8,000 bytes, obscuring their size near the truncation cap.

## What changed

Use logarithmic boundaries through 4 MiB for `codex.additional_context.value_bytes`, with an explicit 4 KiB boundary near the truncation cap.

## Testing

Add a regression test verifying that post-truncation samples between 4,024 and 4,043 bytes land in buckets ending at or below 4 KiB while preserving their sum and count.

GitOrigin-RevId: af268fc0de3d5ceb648ce51fced3918317f2457a

codex-rs/core/src/state/additional_context.rs

@@ -120,7 +120,7 @@ impl AdditionalContextStore {
             telemetry.histogram_with_boundaries(
                 ADDITIONAL_CONTEXT_VALUE_BYTES_METRIC,
                 i64::try_from(bytes).unwrap_or(i64::MAX),
-                ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS,
+                ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS.as_slice(),
                 &size_tags,
             );
         }

codex-rs/otel/src/metrics/names.rs

@@ -75,23 +75,16 @@ pub const THREAD_STARTED_METRIC: &str = "codex.thread.started";
 pub const ADDITIONAL_CONTEXT_COUNT_METRIC: &str = "codex.additional_context";
 /// UTF-8 value bytes before/after truncation, excluding message wrappers; after includes the marker.
 pub const ADDITIONAL_CONTEXT_VALUE_BYTES_METRIC: &str = "codex.additional_context.value_bytes";
-pub const ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS: &[f64] = &[
-    0.0,
-    256.0,
-    512.0,
-    1_000.0,
-    2_000.0,
-    4_000.0,
-    8_000.0,
-    16_000.0,
-    32_000.0,
-    64_000.0,
-    128_000.0,
-    256_000.0,
-    512_000.0,
-    1_024_000.0,
-    4_096_000.0,
-];
+/// Logarithmic byte buckets through 4 MiB, with a 4 KiB boundary near the truncation cap.
+pub static ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS: LazyLock<Vec<f64>> = LazyLock::new(|| {
+    let mut buckets = context_log_buckets(/*max_exponent*/ 22.0)
+        .into_iter()
+        .chain([4096.0])
+        .collect::<Vec<_>>();
+    buckets.sort_by(f64::total_cmp);
+    buckets.dedup();
+    buckets
+});
 pub const THREAD_SKILLS_ENABLED_TOTAL_METRIC: &str = "codex.thread.skills.enabled_total";
 pub const THREAD_SKILLS_KEPT_TOTAL_METRIC: &str = "codex.thread.skills.kept_total";
 pub const THREAD_SKILLS_DESCRIPTION_TRUNCATED_CHARS_METRIC: &str =

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

@@ -3,6 +3,8 @@ use crate::harness::build_metrics_with_defaults;
 use crate::harness::find_metric;
 use crate::harness::histogram_data;
 use crate::harness::latest_metrics;
+use codex_otel::ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS;
+use codex_otel::ADDITIONAL_CONTEXT_VALUE_BYTES_METRIC;
 use codex_otel::Result;
 use codex_otel::THREAD_SKILLS_COUNT_METRIC_BUCKETS;
 use codex_otel::THREAD_SKILLS_DESCRIPTION_TRUNCATED_CHARS_BUCKETS;
@@ -135,6 +137,37 @@ fn histogram_uses_explicit_bucket_boundaries() -> Result<()> {
     Ok(())
 }
 
+#[test]
+fn additional_context_histogram_bounds_truncated_observations() -> Result<()> {
+    let (metrics, exporter) = build_metrics_with_defaults(&[])?;
+    let samples = [4_024, 4_027, 4_043];
+    for value in samples {
+        metrics.histogram_with_boundaries(
+            ADDITIONAL_CONTEXT_VALUE_BYTES_METRIC,
+            value,
+            ADDITIONAL_CONTEXT_VALUE_BYTES_BUCKETS.as_slice(),
+            &[("stage", "after"), ("truncated", "true")],
+        )?;
+    }
+    metrics.shutdown()?;
+
+    let (bounds, counts, sum, count) = histogram_data(
+        &latest_metrics(&exporter),
+        ADDITIONAL_CONTEXT_VALUE_BYTES_METRIC,
+    );
+    assert_eq!((sum, count), (12_094.0, 3));
+    assert_eq!(counts.iter().sum::<u64>(), count);
+    assert_eq!(counts.last(), Some(&0));
+    for (upper_bound, bucket_count) in bounds.iter().zip(&counts) {
+        assert!(
+            *bucket_count == 0 || *upper_bound <= 4_096.0,
+            "post-truncation observations landed in a bucket ending at {upper_bound} bytes"
+        );
+    }
+
+    Ok(())
+}
+
 #[test]
 fn context_histograms_preserve_zero_and_family_ranges() -> Result<()> {
     let (metrics, exporter) = build_metrics_with_defaults(&[])?;