### 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)

self, states: list[TaskInstanceState], session: Session
    ) -> tuple[DefaultDict[str, int], DefaultDict[tuple[str, str], int]]:
        """
        Get the concurrency maps.
        :param states: List of states to query for
        :return: A map from (dag_id, task_id) to # of task instances and
         a map from (dag_id, task_id) to # of task instances in the given state list
        """
        ti_concurrency_query: list[tuple[str, str, int]] = (
            session.query(TI.task_id, TI.dag_id, func.count("*"))
            .filter(TI.state.in_(states))
            .group_by(TI.task_id, TI.dag_id)
        ).all()
        dag_map: DefaultDict[str, int] = defaultdict(int)
        task_map: DefaultDict[tuple[str, str], int] = defaultdict(int)
        for result in ti_concurrency_query:
            task_id, dag_id, count = result
            dag_map[dag_id] += count
            task_map[(dag_id, task_id)] = count
        return dag_map, task_map
    def _executable_task_instances_to_queued(self, max_tis: int, session: Session) -> list[TI]: