### Apache Airflow version

2.5.1

### What happened

I'd expect the `scheduler.tasks.running` metric to represent the number of running tasks, but it is always zero. It appears that #10956 broke this when it removed [the line that increments `num_tasks_in_executor`](https://github.com/apache/airflow/pull/10956/files#diff-bde85feb359b12bdd358aed4106ef4fccbd8fa9915e16b9abb7502912a1c1ab3L1363). Right now that variable is set to 0, never incremented, and the emitted as a gauge.

### What you think should happen instead

`scheduler.tasks.running` should either represent the number of tasks running or be removed altogether.

### How to reproduce

_No response_

### Operating System

Ubuntu 18.04

### Versions of Apache Airflow Providers

_No response_

### Deployment

Official Apache Airflow Helm Chart

### Deployment details

_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)

"""Respond to executor events."""
        if not self._standalone_dag_processor and not self.processor_agent:
            raise ValueError("Processor agent is not started.")
        ti_primary_key_to_try_number_map: dict[tuple[str, str, str, int], int] = {}
        event_buffer = self.job.executor.get_event_buffer()
        tis_with_right_state: list[TaskInstanceKey] = []
        
        for ti_key, value in event_buffer.items():
            state: str
            state, _ = value
            
            ti_primary_key_to_try_number_map[ti_key.primary] = ti_key.try_number
            self.log.info(
                "Executor reports execution of %s.%s run_id=%s exited with status %s for try_number %s",
                ti_key.dag_id,
                ti_key.task_id,
                ti_key.run_id,
                state,
                ti_key.try_number,
            )
            if state in (State.FAILED, State.SUCCESS, State.QUEUED):
                tis_with_right_state.append(ti_key)
        
        if not tis_with_right_state: