feat(asap-precompute-go): SketchEnvelope MetricName/Count/Temporality + Sketch trait split

[zzylol]

Summary

Phase 2 step 2.4b — extends the host-neutral SketchEnvelope and splits the
Sketch trait so the upcoming OTel-shim refactor PRs (steps 2.5–2.9) can
delegate to the runtime without resorting to side-channel labels.

1. SketchEnvelope envelope.go — three new in-process fields

• MetricName string — the metric name the runtime emits each envelope for.
  Lets Adapter.Encode stamp pmetric.Metric.Name() directly instead of
  fishing for the _asap_metric_name label.
• Count uint64 — total observation count this envelope represents (sum of
  per-window samples). Used by the OTel adapter's dp.SetCount() and as a
  sanity field for downstream consumers.
• AggregationTemporality int32 — OTel temporality enum (0 unspecified / 1
  delta / 2 cumulative), stored as int32 not pmetric.AggregationTemporality
  to keep the runtime free of the pmetric import (host-neutral invariant
  from ADR-0002).

These are in-process Go fields only — the proto wire format is unchanged
(Payload []byte is still the canonical sketch state, per ADR-0002 §"Behavior
preservation").

2. Sketch trait split — three new sub-interfaces in precompute.go

The base Sketch interface stays as the runtime's storage type
(Snapshot/ComputeDeltaAgainst/ApplyDelta/Merge/Reset — what window.go and
snapshot_cache.go need). Three sub-interfaces describe what each sketch can
answer:

• QuantileSketch — Quantile(q float64) float64 (DDSketch, KLL).
• CardinalitySketch — EstimateCardinality() float64 (HLL).
• FrequencySketch — EstimateCount(key []byte) float64 and
  TopK(k int) []FrequencyEntry (CountSketch, CountMinSketch).

The runtime never type-asserts to the sub-traits — only adapter encode code
will (e.g. s.(CardinalitySketch) for an HLL gauge).

3. Wire the new envelope fields through the emit path

• PrecomputeConfig gains MetricName string and Temporality int32.
• seriesEntry gains Count uint64, incremented on each scalar Observe and
  accumulated from env.Count on ObserveEnvelope.
• precompute.serializeSeries populates MetricName / Count /
  AggregationTemporality on every emitted envelope.

4. OTel adapter encode — drop the _asap_metric_name label hack

• otel/encode.go::metricNameFor reads env.MetricName directly; the
  lookupLabel helper is removed.
• Doc comment on Encode notes that the labels list no longer carries
  _asap_metric_name since envelopes have a typed field.

Tests

• envelope_test.go (new) — pin zero-value defaults and field assignability.
• precompute_test.go — two new tests confirming envelopes carry MetricName /
  Count / Temporality (populated and zero-config cases), plus compile-time
  var _ CardinalitySketch = (*hllStub)(nil) style assertions for all three
  sub-traits and a runtime smoke test.
• otel/encode_test.go — existing MetricSuffix / MetricName tests updated
  to use the typed MetricName field instead of the legacy label.

Test plan

• cd asap-precompute-go && go build ./...
• cd asap-precompute-go && go test ./...
• cd asap-precompute-go && go test -race ./...
• cd asap-precompute-go && go vet ./...
• OTel-shim refactor PRs (steps 2.5–2.9) build against this without
  reverting to label-hack — verified once those PRs land.

Phase 2

Step 2.4b of the Phase 2 plan: see docs/phase-2-execution-plan.md. Unblocks
the shim refactor PRs (steps 2.5–2.9) which will delegate windowing/snapshot/
delta logic to this runtime instead of duplicating it per-processor.