Merge Queue: New testing capacity metrics

The Merge Queue Prometheus endpoint now exposes separate gauges for active CI testing slots and PRs currently in the testing phase, making batching-aware capacity tracking straightforward.

Two new gauges on the Merge Queue Prometheus endpoint make it easy to track CI capacity separately from the number of PRs under test.

Previously, testing activity was reported as a single number, which conflated CI slot usage with the number of PRs in flight when batching was on. The endpoint now exposes:

• mq_testing_slots_active — number of active CI slots currently in use. A batch counts as one slot, so this maps directly to CI concurrency.
• mq_prs_testing — number of PRs currently in the TESTING state. With batching enabled, this can exceed mq_testing_slots_active because several PRs share a slot.

Use mq_testing_slots_active for CI utilization alerts against your configured concurrency, and mq_prs_testing when you want to see how many PRs are being validated at once.

Both metrics carry the standard repo, branch, and queue_type labels and are available at GET /v1/getMergeQueueMetrics alongside the existing point-in-time gauges and 1-hour window metrics.

See Merge Queue metrics for the full metric reference and example PromQL queries.