Replace OTel metrics SDK with a custom telemetry collector

[ypopovych]

Summary

Replaces the OpenTelemetry metrics SDK behind CI Visibility instrumentation telemetry with a small, purpose-built collector.

The DD telemetry intake wants distributions as raw sample arrays (points: [Double]) and computes the summary server-side — exactly what dd-trace-go sends. The OTel Histogram path bucketed samples and then reconstructed approximate values from bucket midpoints on export: lossy (distorts percentiles) and complex (separate explicit/exponential code paths). Counters/gauges were trivial sums riding the same heavyweight machinery.

This swaps that for a lock-protected MetricStore:

• Counters accumulate a per-interval delta (backend sums deltas).
• Distributions keep every raw sample (backend computes the summary).
• Each drain takes-and-resets the buffers (delta semantics) and writes to the durable TelemetryExporter.

The whole OTel metrics layer — meter provider, views, periodic reader, and the TelemetryMetricExporter bridge — is removed (−1018 lines).

Robustness refinements

• Short flush interval (default 10s) shrinks the window of in-memory metrics a crash can lose. export(item:) persists to disk immediately and upload happens later on the exporter's own schedule, so this adds no network traffic.
• Force-flush on a full distribution buffer so a burst is persisted rather than dropped (cf. dd-trace-go, which drops on a full ring buffer).

Configurable (env-overridable, mirroring DD_TELEMETRY_HEARTBEAT_INTERVAL)

• DD_TELEMETRY_FLUSH_INTERVAL — metric drain cadence (seconds, clamped 1…3600, default 10)
• DD_TELEMETRY_DISTRIBUTION_BUFFER_SIZE — force-flush threshold (samples, clamped ≥ 1, default 65535)

Tag keying

Metrics are keyed by (name, Set<String> tags) — same name + different tags = separate series, matching dd-trace-go (metricKey includes tags). Using a Set makes order-independence intrinsic and dedups; the wire array is sorted once per series at emit time.

Preserved from #274

The app-lifecycle protocol (app-started direct send, app-heartbeat timer, app-closing on shutdown) is kept and now rides the custom collector. Heartbeat keeps its own DD_TELEMETRY_HEARTBEAT_INTERVAL, independent of the metric flush cadence.

Notes

• The metric tree and tag enums are unchanged; only the Counter/Distribution/Factory internals now target the store.
• TelemetryExporter gains a small TelemetryPayloadExporter sink protocol so the collector is unit-testable.
• Tests rewritten against an in-memory payload sink; ARCHITECTURE.md updated.

Test plan

• Framework build succeeds
• Test build succeeds
• TelemetryTests, ConfigTests, DDTracerTests, TelemetryExporterTests pass

🤖 Generated with Claude Code