asap-precompute-go: emit DDSketch relative_accuracy on the wire (fixes empty warm quantile queries)

[zzylol]

Problem

The fused asap_edge warm path emits DDSketch envelopes via oteladapter.Encode, but the encoder never set the output pmetric.DDSketch container's relative_accuracy — unlike the standalone ddsketchprocessor (shim_helpers.go SetRelativeAccuracy). So every fused-edge DDSketch frame shipped relative_accuracy=0.0, and the backend registered the sid as a degenerate ε=0 sketch.

Impact (root-caused live on the Google-cluster trace): the data-plane query router keys backend selection on the accuracy bound. An ε=0 DDSketch reports an exact accuracy bound, so quantile_over_time(...) is routed to the archive (Gorilla/Thanos) instead of the warm SketchStore — and returns empty (kind=exact, no result). The controller policy correctly declared alpha=0.02, but it never reached the wire.

Fix

Thread the alpha from the sketch instance through to the emitted frame:

• DDSketchWrapper.RelativeAccuracy() exposes the configured alpha (sketches/ddsketch.go).
• SketchEnvelope gains an in-process RelativeAccuracy field, populated in serializeSeries from the sketch (0 for non-DDSketch families) (envelope.go, precompute.go).
• otel/encode.go sets dst.SetRelativeAccuracy(env.RelativeAccuracy) on the DDSketch container when > 0.

Tests

otel/relative_accuracy_integration_test.go drives the real DDSketchWrapper through observe → Drain → Encode and asserts alpha=0.02 is preserved on the envelope, the in-memory pmetric.DDSketch, AND a full OTLP proto marshal→unmarshal round-trip — on both the full-snapshot and delta-transmission paths.

End-to-end validation (Google-cluster trace, multinode)

With the fix, a warm p99 query resolves on the SketchStore (not the archive):

quantile_over_time(0.99, gct_ms_ddsketch[2m])  →  status success, data_source=asap_query, latency 2.6ms
  zone z0: 0.04878   zone z1: 0.04878   zone z2: 0.04878   zone z3: 0.05077

Accuracy vs exact offline ground truth — 1.69–2.05% relative error, matching the DDSketch ε=0.02 SLA:

zone	GT p99	warm p99	rel err
z0	0.04962	0.04878	1.69%
z1	0.04981	0.04878	2.05%
z2	0.04974	0.04878	1.93%
z3	0.04993	0.05077	1.69%

Before the fix the same query returned empty (routed to archive as kind=exact).

Notes / follow-ups (out of scope)

• Controller emit currently honors only DDSketch/KLL sketch_family_override; HLL/CMS/CountSketch workload entries are emitted to the agent as ddsketch — a separate controller-emit gap (worked around here with a static 5-family agent config for validation).
• Stacked on feat/sum-aggregation-type; retarget to main after that merges.

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