fix(perf-harness): close 4 gaps from Phase 2.11B deployment perf run

[zzylol]

Summary

Closes the four harness gaps surfaced by Phase 2.11B (PR #238) so the
next operator running the deployment-perf audit gets real numbers in
every column instead of NaN, and so a 10% shim regression is visible
at the CPU level rather than buried in measurement noise.

Per-gap status

#	Gap	Resolution	Where
1	Gateway metric-name skew (v0.108 → v0.141)	FIX	deploy/scripts/measure-baseline.py
2	Backend /metrics empty under ingest-only soak	DOCUMENT	docs/phase-2-perf-deployment.md "Gaps" #2
3	No per-observation latency histogram on shim	FIX (DDSketch only) + follow-up	asap-precompute-go/precompute.go, ddsketchprocessor/{monitor,processor,config_translate,shim_helpers}.go
4	Workload too quiet to discriminate 10% regression	FIX	deploy/docker-compose/baseline-b3-delta.yml

Gap 1 — gateway metric names

Each gateway query is now <v0.141 name> or <v0.108 name>:
otelcol_process_cpu_seconds_total or otelcol_process_cpu_seconds,
otelcol_process_memory_rss_bytes or otelcol_process_memory_rss,
and the analogous _total/no-suffix pair on
otelcol_receiver_accepted_metric_points and
otelcol_exporter_sent_metric_points. Today's stack (v0.141 on both
agent and gateway) populates the first variant; legacy worktree
replays still hit the second. NaN only when neither exists.

Gap 2 — backend /metrics

asap_ingest_samples_total and asap_query_duration_seconds_bucket
genuinely don't exist under ingest-only operation; closing the gap
needs either a PromQL replay path on the harness side or a synthetic
ingest counter on the backend. Both are out of scope for a
harness-fixes PR. Doc updated to call this out plus a docstring note
on the query templates themselves so the operator sees in-script why
the column is blank.

Gap 3 — per-observation latency histogram

Two-part fix:

• Runtime (asap-precompute-go). Adds a LatencyObserver func(d time.Duration) hook installed via Precompute.SetLatencyObserver.
  The hook fires once per Observe call (success, ErrSeriesCapExceeded,
  ErrLateData, matcher-miss all count) so the histogram envelope
  matches what testing.B measured in Phase 2.11A. Stored in an
  atomic pointer so concurrent Observe calls see a coherent snapshot;
  hot-path cost when nil is one Load + nil-check.
• DDSketch shim wiring. enableSelfMonitoring constructs a
  Float64Histogram named asap_processor_observe_seconds with
  bucket boundaries spanning 50 ns – 10 ms. Each per-metric
  Precompute spawned via getOrCreate picks up the histogram via
  proc.recordObserveLatency. Surfaces on the deployed /metrics
  endpoint when EnableSelfMonitoring=true (production default).

The other 4 shims (KLL, HLL, CountSketch, CountMin) stay backwards-
compatible — they don't call SetLatencyObserver and lose nothing.
Wiring those four is mechanical; deferred to Phase 2.11C to keep
this PR focused per the task scope constraint.

Test: TestPrecompute_LatencyObserver in precompute_test.go
exercises hook installation, fires-once-per-Observe semantics
(including post-Tick), and disable.

Gap 4 — saturating workload

baseline-b3-delta.yml now overrides EXPORTER_CARDINALITY=100000
and EXPORTER_FREQ_HZ=100 (both still env-shadowable), chosen to
land the agent in the 50–70% one-core band on the reference
Threadripper hardware. Numerical re-baselining of pre-shim vs
post-shim under the new profile is not in this PR — that's its
own measurement, tracked as Phase 2.11C "saturating-load
comparison".

Validation

cd asap-precompute-go && go test ./...
ok  github.com/ProjectASAP/asap-precompute-go
ok  github.com/ProjectASAP/asap-precompute-go/controlchannel
ok  github.com/ProjectASAP/asap-precompute-go/otel
ok  github.com/ProjectASAP/asap-precompute-go/sketches
ok  github.com/ProjectASAP/asap-precompute-go/telegraf

for p in ddsketch kll hll countsketch countminsketch; do
  cd opentelemetry-collector-contrib-patch/processor/${p}processor && go test ./...
done
ok  .../ddsketchprocessor
ok  .../kllprocessor
ok  .../hllprocessor
ok  .../countsketchprocessor
ok  .../countminsketchprocessor

cd integration/parity && go test -v ./...
ok  github.com/ProjectASAP/ASAPCollector/integration/parity

-race also passes on the new test (atomic-pointer LatencyObserver
slot exercised under concurrent Observe).

Test plan

• go test ./... clean in asap-precompute-go
• go test ./... clean in all 5 shim processors
• go test -v ./... clean in integration/parity
• python3 -c "import py_compile; py_compile.compile('deploy/scripts/measure-baseline.py')" clean
• yaml.safe_load(open('deploy/docker-compose/baseline-b3-delta.yml')) clean
• Re-run the b3-delta soak under the new saturating profile and
  record numbers in a Phase 2.11C doc — out of scope for this PR
  (harness-fixes only)
• Wire the per-Observe histogram into the remaining 4 shims (KLL,
  HLL, CountSketch, CountMin) — Phase 2.11C follow-up

Follow-ups (deferred)

• Phase 2.11C: histogram wiring on KLL/HLL/CountSketch/CountMin
  shims (mechanical copy of the DDSketch monitor.go diff).
• Phase 2.11C: numerical re-comparison of pre-shim vs post-shim
  agent CPU under the saturating workload.
• Standing: asap_sketch_payload_bytes_per_window counter (gap KLL Aggregator Implementation #4
  in the doc, untouched here — separate metric design).
• Standing: deprecated EXPORTER_RATE reference in
  run-baseline-sweep.sh (gap KLL in otel-client #5 in the doc, untouched).
• Standing: PromQL replay path so backend /metrics populates
  under ingest+query soak (gap CountMinSketch in opentelemetry-collector #2 design-level concern).

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