Metrics: Gauge (last-value) via LWWRegister

[keddie]

🤖 This comment was generated by Claude on behalf of @keddie.

Part of #1201.

Gauge — last-value metric via LWWRegister

A gauge reports a current value (temperature, queue depth, in-use memory) — the thing a Sum can't express. The mergeable form is last-value-wins, which is exactly LWWRegister, already in the CRDT zoo. This is the cheapest primitive in the epic: merge by timestamp, done.

Shape

• LWWRegister<Double> (or a numeric payload) per attribute set.
• Merge = higher-timestamp wins (existing LWWRegister semantics).
• Covers OTel Gauge / ObservableGauge (async observed current value).
• Serializes to OTLP NumberDataPoint under a Gauge.

Notes / design surface

• Tie-break must be deterministic — reuse whatever total order LWWRegister already uses (timestamp + replica-id tiebreak); don't introduce a second convention.
• Async/observable gauges are pull-at-collection — the register just holds the latest observed value; no accumulation.
• Confirm the injected Clock/timestamp source is the same one the rest of the exporter uses (time is a dependency — no wall-clock reads).

Acceptance

• Gauge metric backed by LWWRegister, merge-by-timestamp.
• OTLP Gauge/NumberDataPoint serialization.
• Tests: set/observe/merge, last-writer-wins under concurrent updates, deterministic tie-break.

Placement (zoo vs. exporter-local) is resolved once for the epic — see #1201.