MQ-1154 Add metrics() method to Queue binding

[KennethRuan]

Summary

Adds a new metrics() method to the WorkerQueue binding. The method returns consumer backlog information via a GET subrequest to the /metrics endpoint on the upstream Queues service.

Changes

This PR introduces changes to queue.c++ and queue.h to implement the metrics() method. The new method definition is gated behind the queues_metrics_api compat flag and returns the following response type:

type QueueMetrics = {
  backlogCount: number;
  backlogBytes: number;
  oldestMessageTimestamp: number;
};

The upstream changes can be found here:
https://gitlab.cfdata.org/cloudflare/mq/queue-broker-worker/-/merge_requests/1734
https://gitlab.cfdata.org/cloudflare/mq/queue-broker-worker/-/merge_requests/1759

Testing

• bazel test //src/workerd/api/tests:queue-test@ - queue send/sendBatch tests + error-codes tests pass
• bazel test //src/workerd/api/tests:queue-test@all-compat-flags
• bazel test //src/workerd/api/tests:queue-metrics-test@ - tests new api with flag enabled + flag disabled
• bazel test //src/workerd/api/tests:queue-metrics-test@all-compat-flags

[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[KennethRuan]

I have read the CLA Document and I hereby sign the CLA