feat(router): observe subscription delivery outcomes

[mwisner]

Summary

• add unsampled stream-processing counters for messages dispatched or removed by pre-dispatch hooks
• add subscription resolution-error counters with finite reason tags
• add WebSocket frame outcome counters that distinguish data, partial/error payloads, terminal errors, completion, success, and failure
• promote WebSocket write failures to structured warning logs

Why

router.engine.messages.sent combines successful data updates, resolver error paths, and heartbeats. It cannot show whether an update was data or an error, and several resolver-error paths increment it without knowing whether the error frame reached the WebSocket. A failed normal data flush is also not represented by that counter.

The new router.engine.websocket.frames counter records the actual protocol write result. This distinguishes, for example:

• successful data delivery: frame_type:data, result:success
• a resolver error successfully reported to the client: resolution error plus an errors_only frame with result:success
• a resolver error whose error frame could not be written: resolution error plus result:failure
• successful resolution followed by a disconnected/broken client: data frame with result:failure

A successful WebSocket write is server-side delivery, not proof that the client application processed the frame.

Branch relationship

This is a stacked PR based on mwisner/fix/kafka-consumer-client-leak, the head branch for wundergraph#3133. The branch therefore retains the merged-context Kafka poller and deferred consumer-client cleanup. This PR must not be merged into the leak-fix branch; after wundergraph#3133 merges, it should be retargeted/rebased onto upstream main.

No StockX custom-module code or custom-module observability interface is involved.

Metrics

• router.streams.processed.messages
  • wg.stream.processing.result: dispatched or dropped
  • wg.stream.processing.reason: hook error, timeout, panic, removal, or none
  • provider, destination, and GraphQL root-field attributes
• router.engine.subscription.resolution.errors
  • wg.subscription.reason
• router.engine.websocket.frames
  • frame type, payload type, result, and failure reason

Validation

• go test -race ./...
• go vet ./...
• staticcheck ./...
• ancestry check against PR fix(router): close Kafka consumer client when subscription ends wundergraph/cosmo#3133 head b3f25fd276ea5c3cfdad36010e2956f49e4fd43d

[github-actions]

Router image scan passed

✅ No security vulnerabilities found in image:

ghcr.io/mwisner/cosmo/router:sha-8a3547f7fa667a70e7d1e66c224ea4ef0a57762d