Middleware and observability

Middleware

Mux.Use wraps every handler (logging, panics-to-errors, tracing); NewClient's variadic EnqueueMiddleware wraps every insert the same way (e.g. to stamp tenant metadata).

Observability

kilnotel traces every job from the request that enqueued it to the handler that ran it, and records job counts, durations and the delay between a job's scheduled time and its start:

client := kiln.NewClient(store, kilnotel.EnqueueMiddleware())
mux.Use(kilnotel.Middleware())
unregister, err := kilnotel.Observe(server, store)

It depends only on the OpenTelemetry API, so it reports through whatever SDK and exporters the application already has. Server.Stats() and Server.Healthy() cover the same ground without it.