feat(telegraf-patch): allsketches processor (Phase 4 step C)

[zzylol]

Summary

Implements processors.allsketches per docs/design-asap-telegraf-integration.md §3, §5, §6, §8 — a single unified Telegraf StreamingProcessor whose sketch_type config field dispatches into the existing asap-precompute-go/sketches/<type> wrappers. No build script, no submodule patch — those are Phase D.

File layout

telegraf-patch/processors/allsketches/:

• allsketches.go (203 LoC) — StreamingProcessor impl: Start validates config + spawns flush ticker, Add decodes via Phase-B codec then calls Precompute.Observe, Stop drains pending window with now+2*window to force-rotate the tumbling window.
• config.go (156 LoC) — toPrecomputeConfig / toAdapterConfig / sketchFactory dispatch table mapping sketch_type → wrapper constructor + observer.
• factory.go (27 LoC) — processors.AddStreaming("allsketches", …) registration with codec defaults.
• sample.conf (33 LoC) — canonical [[processors.allsketches]] block.
• README.md (121 LoC) — user-facing config docs.
• allsketches_test.go (204 LoC) — lifecycle coverage.
• go.mod / go.sum — module rooted at github.com/influxdata/telegraf/plugins/processors/allsketches, with sibling-checkout replace directives for asap-precompute-go, sketchlib-go, and the patched pdata.

Dispatch table

sketch_type	Factory	Observer
ddsketch	sketches.NewDDSketchWrapper(alpha)	sketches.DDSketchObserver{}
kll	sketches.NewKLLWrapper(k, seed)	sketches.KLLObserver{}
hll	sketches.NewHLLWrapper()	sketches.HLLObserver{}
countsketch	sketches.NewCountSketchWrapper(rows, cols)	sketches.CountSketchObserver{DefaultKey}
countminsketch	sketches.NewCMSWrapper(rows, cols, false)	sketches.CMSObserver{}

Defaults applied per sketch when unset: ddsketch alpha=0.01, kll k=200, countsketch/cms rows=4 cols=2048.

Test summary

go test -race ./... — all green (1.145s):

• TestStart_InvalidSketchType — unknown sketch_type rejected at Start.
• TestStart_InvalidWindow — unparsable window_size rejected at Start.
• TestDDSketch_HappyPath — Start → 32 scalar Adds → Stop emits envelope-bearing metrics on the accumulator; verifies _asap_envelope_b64 field is base64-decodable.
• TestEnvelopeShortcut — feed pre-aggregated _asap_envelope_b64 field, decode-side envelope-shortcut path exercised without value_field.
• TestStopWithoutStart — Stop is a safe no-op when Start was never called (Telegraf reload race protection).
• TestAllSketchTypes — loops over all five sketch_types, each constructs and processes 8 scalars and emits at least one envelope on Stop.

Test plan

• go build ./... clean
• go vet ./... clean
• go test -race ./... all pass
• Phase D: build_sketchtelegraf.sh builds binary including the registered plugin
• Phase E: cross-host envelope parity vs sketchcollector

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