feat(kllprocessor): emit typed KLLSketchDataPoint

[zzylol]

Last of the four processor typed-emission refactors (#158: countmin, #159: countsketch, #160: hll dead-code cleanup, this PR: kll). Replaces Gauge-with-byte-attribute emission with typed KLLSketchDataPoint messages so ASAPQuery-backend's modified-OTLP sketch router sees them as Metric.data = KLLSketch{...} variants instead of anonymous Gauges.

Why

Before this PR the TransmitSketch path called appendKLLSketchDataPoint which emitted:

dp := metric.Gauge().DataPoints().AppendEmpty()
dp.Attributes().PutInt(\"kll.k\", int64(k))
dp.Attributes().PutInt(\"kll.count\", int64(sketch.Count()))
dp.Attributes().PutEmptyBytes(\"kll.sketch_payload\").FromRaw(payload)
dp.SetDoubleValue(float64(sketch.Count()))

ASAPQuery-backend's modified-OTLP decoder matches on Metric.data = KLLSketch{...} (oneof tag 14, typed KLLSketchDataPoint). The Rust-side KLL decoder (ASAPQuery #8 §KLL, with lossy statistical reconstruction via item replay) was already wired up, but no processor produced the typed data points it wanted. Processor and decoder were both ready — just speaking past each other on the wire.

What changed

processor.go

• Batch path — TransmitSketch branch now calls findOrCreateKLLSketchMetric (creates a typed pmetric.MetricTypeKLLSketch metric with AggregationTemporalityDelta) and appendTypedKLLSketchDataPoint (writes SetSketch / SetEncoding / SetCount on a typed KLLSketchDataPoint).
• Windowed path — same two helpers. Inline m.SetEmptyGauge() replaced with m.SetEmptyKLLSketch().SetAggregationTemporality(...).
• New helper findOrCreateKLLSketchMetric mirrors findOrCreateGaugeMetric but creates typed KLL metrics and matches on MetricType so legacy Gauge metrics with the same name don't get accidentally reused.
• New helper appendTypedKLLSketchDataPoint replaces the old appendKLLSketchDataPoint. Writes the sketch payload via SetSketch and tags the encoding with KLLSketchEncodingProto. Keeps kll.k on the attribute map for operator visibility. Sum/Min/Max fields are left at zero because sketchlib-go's KLLSketch doesn't track them — KLL is quantile-only.
• Old appendKLLSketchDataPoint deleted. Old findOrCreateGaugeMetric retained because the non-TransmitSketch quantile-emission path still exports gauge-shaped scalar quantile series.

processor_test.go

TestBatchMode_TransmitSketch previously asserted on m.Gauge().DataPoints().At(0) and read the payload from the kll.sketch_payload byte attribute. Updated to assert m.Type() == pmetric.MetricTypeKLLSketch, read outDP.Sketch() directly, and verify outDP.Encoding() == pmetric.KLLSketchEncodingProto.

Other tests in the file target the quantile-emission path (TransmitSketch: false), which still uses Gauge — unaffected.

Validation

Same pre-existing go.opentelemetry.io/collector/processor/selfmonitor resolution issue as #158/#159/#160 blocks local go build.

• ✅ gofmt -l: clean on both modified files
• ✅ All API methods used (pmetric.MetricTypeKLLSketch, SetEmptyKLLSketch, KLLSketch().DataPoints().AppendEmpty(), SetSketch, SetEncoding, SetCount, KLLSketchEncodingProto) already exist on the pmetric patch

Stack

• #158 (merged) — countminsketchprocessor typed emission
• #159 — countsketchprocessor typed emission
• #160 — hllprocessor dead-code cleanup
• This PR — kllprocessor typed emission
• Next — per-processor MSGPACK config option (one-line branch each). Note: for KLL specifically, sketchlib-go #50/#51 out-of-scope'd the msgpack wire format because sketchlib-go's KLL and sketch-core's KLL don't share a byte-level backend. KLL producers should keep using _ENCODING_PROTO; the MSGPACK enum value in #157 is reserved but not usable for KLL today.

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