feat(asap-precompute-go): telegraf codec (Phase 4 step B)

[zzylol]

Summary

Phase B of the Telegraf integration plan in
docs/design-asap-telegraf-integration.md:
the asap-precompute-go/telegraf/ codec. Pure data-shape translation
between telegraf.Metric and the host-neutral
Observation / SketchEnvelope, mirroring asap-precompute-go/otel.

In scope

• decode.go — Decode(metric, cfg) (*Observation, error). Reads
  cfg.ValueField as a float (KindFloat), or routes through
  cfg.EnvelopeField (default _asap_envelope_b64) as a
  pre-aggregated SketchEnvelope shortcut (KindEnvelope). Returns
  typed errors for missing / non-numeric value fields and malformed
  envelope payloads. Tags are sorted by key for stable series-key
  hashing — same contract as the OTel codec.
• encode.go — Encode([]*SketchEnvelope, cfg) ([]telegraf.Metric, error).
  One emitted metric per envelope; env.MetricName as Name (with
  cfg.OutputMetricName fallback); env.ResourceLabels flattened into
  Tags alongside env.Labels (last-write semantics on collision so
  data-point Labels win); single field whose value is
  base64(JSON(SketchEnvelope)); timestamp from WindowEndMs
  (falling back to WindowStartMs, then time.Now()).
• adapter.go — thin Adapter struct wrapping the package-level
  helpers; mirrors otel.Adapter shape but does NOT implement
  precompute.Adapter (no goroutines / tickers — those live in the
  Phase C plugin).
• config.go — AdapterConfig with ValueField, EnvelopeField,
  OutputMetricName plus DefaultAdapterConfig().

Out of scope (deferred)

• Phase C — the telegraf-patch/processors/allsketches/ plugin
  (StreamingProcessor, flush ticker, control-channel goroutine,
  config translation). Not touched in this PR.
• Phase D — build_sketchtelegraf.sh + Telegraf submodule
  registration patch.
• Phase E — cross-host envelope parity test against
  sketchcollector.
• Multi-field metric fanout (single value_field only per design doc
  §10).
• Custom asap Telegraf serializer (separate PR per design §4).

Test coverage

go test ./telegraf/... runs 22 tests covering:

• Decode happy path: float scalar → KindFloat.
• Decode int64 → cast to float.
• Decode custom value field.
• Decode missing value field → typed error.
• Decode non-numeric value field → typed error.
• Decode nil metric → error.
• Decode envelope shortcut: base64-JSON envelope field → KindEnvelope,
  scalar field ignored.
• Decode malformed envelope → wrapped error.
• Decode with empty envelope-field config → shortcut disabled, falls
  back to scalar path.
• Decode with nil config → uses package defaults.
• Encode happy path: envelope → metric with base64-JSON field, tags,
  timestamp from WindowEndMs.
• Encode metric-name fallback to cfg.OutputMetricName.
• Encode flattens ResourceLabels into Telegraf tags.
• Encode label / resource-label key collision: Label wins.
• Encode nil envelope → indexed error.
• Encode empty slice → nil slice, no error.
• Encode empty EnvelopeField config → error (data-loss prevention).
• Encode no timestamp → time.Now().
• Encode WindowStartMs fallback when WindowEndMs is zero.
• Encode field type-check (always plain string).
• Adapter Decode / Encode wrap helpers.
• Adapter constructor preserves config pointer.
• Round-trip: Encode → Decode produces equivalent envelope (full
  field-by-field assertion).

go build ./..., go vet ./..., go test ./... all green; existing
otel/ and runtime tests unchanged.

Dependencies

Adds github.com/influxdata/telegraf v1.34.4 as a direct dep (pinned
to a release that matches the project's existing go 1.24.0
directive — newer 1.36+ telegraf releases require Go 1.25+, which
would force a toolchain bump that's out of scope here). Tidy left the
indirect-dep tree unchanged except for one zeebo/assert patch bump
that telegraf brought along — net diff is 1 require line in
go.mod and ~6 lines of go.sum. Telegraf does NOT pull in a heavy
transitive tree at this code-import depth (we only use
telegraf.Metric and metric.New).

Test plan

• go build ./... from asap-precompute-go/
• go vet ./...
• go test ./telegraf/... (22 PASS)
• go test ./... (all packages green; existing otel/ /
  controlchannel/ / sketches/ tests still pass)
• Phase C will exercise the codec from a real Telegraf
  StreamingProcessor — that's where end-to-end correctness is
  proven; this PR alone doesn't gate on a sketchtelegraf binary
  build.

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