feat(asapedgeprocessor): accept per-metric sample_p in fused asap_edge + sample HLL/CMS

[zzylol]

Summary

• Activating sampling crash-looped the edge collector: the control plane emits a per-metric sample_p into the FUSED asap_edge processor's config (set when a metric's workload sample_p < 1), but the fused per-metric config struct didn't define the key, so the collector's strict mapstructure decode rejected it (metrics[N] has invalid keys: sample_p) and crashed. A prior PR (feat(sketches): per-metric sample_p knob on the CMS + HLL warm processors #441) added SampleP only to the NON-fused hll/countminsketch processors; the deploy runs ASAP_EDGE_FUSED=1.
• Add SampleP float64 \mapstructure:"sample_p"`toMetricFamily(the fused per-metric struct) inconfig.go, validated to (0,1]with0/unset normalised to 1.0` (sampling disabled).
• Wire it into the warm sketch-build (sketch.go newSketchAggregator): the HLL and CountMinSketch factories call sketchlib-go's WithSampleP(p). WithSampleP(1.0) is an exact no-op, so the unset/1.0 path stays byte-identical to today. DDSketch/KLL/CountSketch (no sampling support) ignore it. Mirrors the standalone processors' wiring from feat(sketches): per-metric sample_p knob on the CMS + HLL warm processors #441.
• Backend already reads sample_p off the SketchEnvelope and rescales by 1/p at query, so the agent just needs to accept the key and build the sketch with the probability.

Files / functions touched (for merge-coordination with the cold-density agent)

Scoped to the warm path only — the cold/gorilla feed path is untouched:

• config.go — MetricFamily struct (+SampleP field); Config.Validate() (+(0,1] normalise/validate loop).
• sketch.go — newSketchAggregator() (apply WithSampleP to HLL/CMS factories; retain factory on sketchAggregator for test observability).
• sample_p_path_test.go — new test file.
• go.mod/go.sum — add confmap (test-only, for the strict-decode regression).

Test plan

• go build ./..., go vet ./..., gofmt -l clean
• New sample_p_path_test.go:
  • TestFusedConfigSamplePDecodes — a fused config with sample_p on HLL/CMS metrics decodes through the real strict confmap path WITHOUT the crash; values populate; unset normalises to 1.0.
  • TestFusedSketchBuildAppliesSampleP — the built HLL/CMS sketch has SampleP()==0.5 with sample_p: 0.5, and 1.0 (no sampling) when unset; DDSketch ignores the knob.
  • TestFusedConfigSamplePRejectsOutOfRange — out-of-range sample_p rejected at Validate.
• Full package suite passes (no pre-existing processor_test.go pdata-skew in this checkout)

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