dag_processing.processes metric missing decrement on graceful process exit

### Under which category would you file this issue? Airflow Core ### Apache Airflow version 3.3.0 ### What happened and how to reproduce it? The dag_processing.processes metric description states: "the delta is negative when, since the last metric was sent, processes have completed." However, `_collect_results()` , the normal graceful completion path, emits no stats.decr, so the delta is never negative under normal operation. A decr is only emitted for abnormal exits , which are : stop (file deleted), timeout, and terminate (shutdown). This means the metric does not behave as documented. **Current behaviour** `stats.incr("dag_processing.processes", tags={"action": "start", ...})` is called in `_start_new_processes()` when a child `DagFileProcessorProcess` is spawned. A matching `stats.decr(...)` is called in three abnormal exit paths: - `terminate_orphan_processes()` action: `"stop"` (file deleted mid-parse) - `_kill_timed_out_processors()` action: `"timeout"` (exceeded `processor_timeout`) - `terminate()` action: `"terminate"` (manager shutting down) However, `_collect_results()` , the **normal graceful exit path** called every loop iteration pops the finished processor from `self._processors` and calls `processor.close()` with **no corresponding `decr`**: This means under normal operation (parsers completing successfully), the metric value drifts upward indefinitely and never accurately reflects the number of currently running processes ### What you think should happen instead? A `stats.decr("dag_processing.processes", tags={"action": "finish", ...})` should be emitted when a process completes gracefully, symmetric with the incr in `_start_new_processes()`. ### Operating System _No response_ ### Deployment None ### Apache Airflow Provider(s) _No response_ ### Versions of Apache Airflow Providers _No response_ ### Official Helm Chart version Not Applicable ### Kubernetes Version _No response_ ### Helm Chart configuration _No response_ ### Docker Image customiza…