- Restructure: writers.ts → writers/json.ts; combine OTLP mapping and
  writer into writers/otlp.ts; lift signal classification into
  export/signal.ts.
- Group all records of each signal under a single Resource block per
  file (was one Resource per event), matching the OTLP spec's
  recommended shape and shrinking exports at scale.
- Adopt OTLP enums from @opentelemetry/api + api-logs; document the
  +1 wire offset on SpanKind.
- Tighten the public surface: only writeOtlpZipExport and
  OtlpExportCounts are exported; record mappers are module-private.
- Hoist per-event metric attributes once; single-pass partition of
  http.requests measurements into counts and gauges.
- Share parseTelemetryTimestampMs between range.ts and the OTLP
  writer's nanos conversion (was duplicated with identical error).
- Extract asyncIterable test helper to test/mocks/; inline the memfs
  vi.mock pair across affected test files.
- Tests now exercise the public API only, dropping side-effect
  assertions about staging cleanup and atomic semantics already
  covered by writeAtomically's own tests.

Wire-format fixes:
- Switch http.requests count_* sums from DELTA to CUMULATIVE temporality
  (Prom/Mimir/Grafana Cloud reject the delta+sum combination).
- Maintain per-series running totals across the export, anchored at the
  first observed window's startTimeUnixNano.
- Suppress zero-cumulative counters so routes that never errored don't
  ship empty data points.

Semantic-convention compliance:
- Use OTel-standard `service.instance.id` (was `coder.session.id`) and
  `host.id` (was `coder.machine.id`) for portability with OTel-aware
  backends. The previous vendor keys were pure aliases.
- Add `schemaUrl` to each ResourceLogs/ResourceSpans/ResourceMetrics
  container and each scope container.
- Stamp scope.version with the extension version.

Group OTLP code into src/telemetry/export/writers/otlp/ with one file per
concern, and hoist metric classification to src/telemetry/export/metrics.ts
so it can be shared by future writers.

Layout:
- metrics.ts:     domain (isMetricEvent, describeMetricEvent, units)
- otlp/writer.ts: public API + orchestration (signal routing, packZip)
- otlp/envelope.ts: streaming JSON envelope writer + error wrapping
- otlp/records.ts:  log/span/metric record builders + helpers
- otlp/types.ts:    OTLP wire-format interfaces (pinned to proto v1.10.0)

Safety fixes surfaced by the layer split:
- envelope: listen for stream 'error' events so failed opens reject pending
  writes instead of hanging; close() is idempotent; suffix-write failures
  during close are labeled as close failures.
- writer:   on loop failure, use Promise.allSettled to close streams without
  masking the original error.
- records:  coerce NaN/Infinity numeric inputs to 0n instead of throwing
  from BigInt(); clamp startTimeUnixNano <= timeUnixNano for out-of-order
  events; include eventName in the cumulative-series key.

Each layer has its own test file; total grows from 99 to 131 cases.