perf(ingest): gzip and sample the gateway's own telemetry - #1214

🟢 Confidence 4/5 · likely safe to merge
The sampler's weighting and the parent-based decision path are unit-tested and consistent with SAMPLE_RATE_EXPR; the only lasting risk is the prd volume cut, which is config-driven.
_{quality 100/100 · no findings · tests covered · risk medium}

The gateway's four OTLP exporters now gzip, and its own traces are head-sampled by a new SelfTraceSampler driven by INGEST_SELF_TRACE_SAMPLE_RATIO (prd 0.05), which stamps SampleRate so warehouse throughput stays weighted. Safe to merge.

• SelfTraceSampler wraps ParentBased(TraceIdRatioBased) and stamps SampleRate = 1/ratio on sampled spans
• INGEST_SELF_TRACE_SAMPLE_RATIO goes through parse_sample_ratio, refused outside (0, 1]
• All four OTLP exporters set OtlpCompression::Gzip; gzip-http added to Cargo features
• prd sets 0.05 via resolveIngestSelfTraceSampleRatio; log and metric export stay complete

Fixed since the last review

• ✅ F1 · with_sampler overrides the documented OTEL_TRACES_SAMPLER knob

_{de19115 · Updated on every push. Reply "won't fix" to dismiss a finding, or mention @maple-review-bot to ask about one.}