## Why
Exec-server request and connection latencies need fractional-second
histograms. The existing duration API records integer milliseconds and
uses millisecond-scale buckets.
## What changed
- Adds a described duration API that records `Duration` values as
fractional seconds.
- Uses second-scale explicit histogram boundaries.
- Caches duration histograms by name, unit, and description, matching
the existing instrument caching model.
- Covers exact boundaries, representative bucket placement, fractional
sums, and exported metadata.
This PR only adds the duration primitive. It does not add exec-server
adoption.
## Stack
1. #26091: counter descriptions
2. #27057: gauge instruments
3. **#27058: second-based duration histograms**
4. #25019: initialize exec-server OpenTelemetry at startup
Related independent coverage: #27059 tests OTLP HTTP log and trace event
export.
## Validation
- `just test -p codex-otel`
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