feat: add extra observability metrics for PrometheusMiddleware and Receiver

[Mohammed0tarek]

Summary

• PrometheusMiddleware: add queue_wait_seconds histogram (end-to-end queue latency via _taskiq_enqueue_timestamp label injected in pre_send), task_errors_by_type counter
  (error count broken down by exception class name), and idempotent timestamp injection so multiple middlewares can coexist
• ReceiverObserver protocol: a new typing.Protocol (taskiq/receiver/observer.py) that lets external implementations observe receiver internals — prefetch queue depth, semaphore
  availability, active task count, unknown task lookups, and deserialization errors — with zero overhead when no observer is set (if self.observer is not None guards)
• PrometheusReceiverObserver: concrete implementation using Prometheus Gauges/Counters, wired automatically via PrometheusMiddleware.set_broker() → broker._receiver_observer →
  Receiver(observer=...)

New metrics

Metric	Type	Source
queue_wait_seconds	Histogram	PrometheusMiddleware
task_errors_by_type	Counter (task_name, error_type)	PrometheusMiddleware
prefetch_queue_size	Gauge	PrometheusReceiverObserver
semaphore_available	Gauge	PrometheusReceiverObserver
worker_active_tasks_count	Gauge	PrometheusReceiverObserver
task_not_found_total	Counter (task_name)	PrometheusReceiverObserver
deserialize_error_count	Counter	PrometheusReceiverObserver

Some decisions decisions

• ReceiverObserver is a Protocol (not ABC) so there's no coupling — any class with matching methods satisfies it
• queue_wait_seconds gracefully degrades: if the sender doesn't use PrometheusMiddleware, the timestamp label is absent and the metric is simply not observed
• Negative queue wait values (from clock skew) are clamped to 0. If needed we can change this behavior.

Testing

Honestly still no idea how to test this via py test. But I'll be working on it.