dunglas / mercure / 25374805339

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feat(tracing): OpenTelemetry spans for core operations (#1211)

* feat(tracing): instrument publish, subscribe, subscriptions, and Bolt history with OpenTelemetry spans

Spans are obtained via trace.SpanFromContext(ctx).TracerProvider(), so
they nest under the parent span carried in the request context — in
particular the HTTP span produced by Caddy's `tracing` directive, which
already honors the standard OTEL_* environment variables.

When no parent span is active, the lookup yields the noop tracer, so
there is no runtime cost and no globals are touched.

* chore(tracing): fix CI, address review feedback

- tracer.go: silence ireturn/spancheck linters (trace.Span is an
  interface by design).
- subscription.go: deduplicate the two handler prologues via a
  refactored initSubscription that starts the span itself, and downgrade
  the span kind to Internal per OpenTelemetry conventions (Caddy's
  tracing directive already produces the outer HTTP server span).
- publish.go / subscribe.go: gate span.SetAttributes calls on
  span.IsRecording() so no attribute slices are allocated when tracing
  is disabled.
- Lint nolint bumps (gocognit, funlen, nestif, contextcheck) where the
  new error-handling added one extra level of nesting but the logic is
  intentionally local.
- docs/hub/{metrics,tracing}.md: Markdown tables reformatted by
  prettier; subscriptions kind updated to Internal.

* chore(tracing): restore ctx := r.Context() pattern in subscription handlers

initSubscription no longer propagates the span context through
r.WithContext; the span is returned and ended by the caller instead.
This keeps the handlers' r untouched, so the existing ctx := r.Context()
pattern works as it did on main and contextcheck stays happy.

* chore(lint): allow trace.Span returns in ireturn

Configure the ireturn linter to allow go.opentelemetry.io/otel/trace.Span
as a permitted interface return type, and drop the per-function
//nolint:ireturn directives from startSpan and... (continued)

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