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 customizations

_No response_

### Anything else?

_No response_

### Are you willing to submit PR?

- [x] Yes I am willing to submit a PR!

### Code of Conduct

- [x] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

Start by reading _start_new_processes() and _collect_results(), then compare the existing decr calls in terminate_orphan_processes(), _kill_timed_out_processors(), and terminate(). Done means graceful processor completion emits a matching stats.decr with action "finish" and the relevant DAG processing metric tests pass.

Written by the indexing model from the issue text.