Better observability for run times and worker utilization

We run self hosted Spacelift and would like the following observability.

Run Execution Metrics

End-to-end run duration (histogram): Distribution of total wall-clock time from run creation to terminal state. Enables tracking p50/p90/p99 durations, detecting regressions, and setting SLOs on deployment latency. Should support labels for stack, space, run type, and terminal state.


Worker Pool Metrics

Runs queued for workers (gauge): Count of runs currently waiting for a worker, queryable over time. Enables alerting on queue saturation and right-sizing worker pools. Should support labels for stack, space, and worker pool.

Per-run worker wait time (histogram): Distribution of time each run spends waiting for a worker before execution begins. Should support labels for stack, space, and worker pool.