fix(gateway): unbreak boot post-#206 + simplify merge config

[zzylol]

Summary

Caught when running the e2e for the first time after #206 merged. Two boot bugs in the gateway configs land here, plus a scope-correction on the "DD/KLL/HLL merge processors follow-up" I named in #206.

1. Gateway didn't boot — prometheusremotewriteexporter not in the patched build

The OCB manifest at opentelemetry-collector-contrib-patch/cmd/sketchcollector/builder-config.yaml ships only otlpexporter, otlphttpexporter, debugexporter, prometheusexporter — no PRW. gateway.yaml's "non-active fallback" PRW block fails the config decode on startup:

'exporters' unknown type: "prometheusremotewrite"
  (valid values: [otlphttp prometheus debug otlp])

Removed the block. Comment in its place documents that adding PRW would need a manifest edit + rebuild.

2. service.telemetry.metrics.address is deprecated in v0.141

v0.141 only accepts the readers: form (the agent configs already use it). Same fix applied in all three gateway configs:

metrics:
  level: detailed
  readers:
    - pull:
        exporter:
          prometheus:
            host: 0.0.0.0
            port: 8890

3. The DD/KLL/HLL "merge processors" follow-up I named in #206 isn't needed

I had this wrong in the #206 description. The existing five sketch processors already handle merge for the typed wire format — each one's input switch dispatches both raw inputs AND typed sketch inputs into the same windowed accumulator. Code references:

• processor/countminsketchprocessor/processor.go:269 — case pmetric.MetricTypeCountMinSketch: decodes incoming typed CMS payloads via inboundDecodeCMS.
• processor/ddsketchprocessor/processor.go:206 — consumeDDSketchDataPoints decodes incoming typed DDSketch payloads.

Same in the other three. Configuring the regular ddsketch processor at the gateway with the same window the agent used produces a windowed cross-agent merge. No new merge processors needed.

The legacy countminsketchmergeprocessor / countsketchmergeprocessor handle a DIFFERENT wire format (Gauge data points with sketch_payload byte attributes — pre-typed-Metric.data). They're not used by today's typed-wire e2e.

gateway-aggregate-from-sketches.yaml now uses ddsketch (same as agent) with KLL / HLL / CS / CMS commented in for opt-in, plus a comment block explaining the dual-mode dispatch.

e2e probe

With the patched gateway up and these fixes applied, sent a test OTLP probe (HTTP, Gauge metric test_probe) from inside the docker network. It traversed:

• Gateway OTLP HTTP receiver: transport="http" accepted_metric_points=1
• Gateway batch processor: 1 batch, send size 1
• Gateway OTLP exporter to backend: sent_metric_points=1
• Backend OTLP gRPC receiver: OTLP ingest: received 1 resource metrics, 1 total data points (transport=gRPC)
• Backend ingest router: OTLP parse example: test_probe {} @1714525000000000000ns = 42

End-to-end OTLP round-trip works, confirming the patched-gateway preservation is correct. The typed-sketch path uses the same OTLP framing → inherits the preservation.

(Aside, not in scope for this PR: separately observed the agent's [ddsketch, batch] pipeline producing output_metric_points_total=4000 from ddsketch but no exporter_sent_metric_points_total on the wire — looks like an agent-side pipeline-wiring issue between ddsketch and batch processors. Orthogonal to the gateway preservation. Worth a separate look.)

Test plan

• gateway.yaml parses against patched build's component set (no PRW component).
• All three gateway configs use the v0.141 readers: telemetry syntax.
• gateway-aggregate-from-sketches.yaml uses ddsketch from the regular processor set, not the legacy merge processors.
• Stack comes up cleanly with GATEWAY_CONFIG=gateway.yaml (default).
• Probe traversal confirmed end-to-end (test_probe Gauge through agent → gateway → backend).
• Real sketch traffic verification deferred — the agent's [ddsketch, batch] flow has an unrelated issue.

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