## Why
Metric descriptions should be declared with reusable OTEL instruments
instead of being coupled to individual consumers. Counter descriptions
are the smallest API primitive needed by the exec-server observability
work.
## What changed
- Adds `counter_with_description` while preserving the existing counter
API.
- Caches counters by name and description so instrument metadata remains
part of the declaration identity.
- Covers the exported description together with the existing value and
attribute contract.
This PR only adds counter descriptions. It does not add gauges,
second-based durations, 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.
The `codex-exec-server` bounded service tag now stays with the
exec-server adoption change instead of this reusable infrastructure
stack.
## Validation
- `just test -p codex-otel`
- `just fix -p codex-otel`
- `just fmt`
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