Add scheduler.dagruns.not_started gauge metric for late-starting scheduled DagRuns

[takayoshi-makabe]

Add a new scheduler.dagruns.not_started gauge metric that tracks scheduled DagRuns that have not started within a configurable threshold. Users can alert on scheduling latency directly via StatsD/OpenTelemetry without writing custom DB queries.

The threshold is configurable via [scheduler] dagrun_late_threshold (default 15.0 seconds), which mirrors the default used by Prefect's MarkLateRuns service (PREFECT_API_SERVICES_LATE_RUNS_AFTER_SECONDS).

DB impact:

Two COUNT(*) queries are added to the existing dagrun_metrics_interval (default 30 s) polling cycle, following the same pattern as _emit_running_dags_metric. The dag table query uses idx_next_dagrun_create_after; the dag_run query relies on the existing idx_dag_run_queued_dags index.

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