## Why
Exec-server observability needs current-value measurements in addition
to counters. The reusable OTEL client should expose that primitive
without coupling it to exec-server runtime behavior.
## What changed
- Adds integer gauge instruments, with optional descriptions.
- Caches gauges by name and description so instrument metadata remains
part of the declaration identity.
- Covers gauge values, descriptions, merged attributes, and OTLP HTTP
export.
This PR only adds the gauge primitive. It does not add second-based
duration histograms or exec-server adoption.
## Stack
1. #26091: counter descriptions
2. **#27057: gauge instruments**
3. #27058: second-based duration histograms
Related independent coverage: #27059 tests OTLP HTTP log and trace event
export.
## Validation
- `just test -p codex-otel`
- `just fix -p codex-otel`
- `just fmt`
7e5e41daea