### 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,
        dag_run: DagRun,
        session: Session,
    ) -> DagCallbackRequest | None:
        """
        Make scheduling decisions about an individual dag run.
        :param dag_run: The DagRun to schedule
        :return: Callback that needs to be executed
        """
        callback: DagCallbackRequest | None = None
        dag = dag_run.dag = self.dagbag.get_dag(dag_run.dag_id, session=session)
        dag_model = DM.get_dagmodel(dag_run.dag_id, session)
        if not dag or not dag_model:
            self.log.error("Couldn't find DAG %s in DAG bag or database!", dag_run.dag_id)
            return callback
        if (
            dag_run.start_date
            and dag.dagrun_timeout
            and dag_run.start_date < timezone.utcnow() - dag.dagrun_timeout
        ):
            dag_run.set_state(DagRunState.FAILED)
            unfinished_task_instances = (
                session.query(TI)
                .filter(TI.dag_id == dag_run.dag_id)
                .filter(TI.run_id == dag_run.run_id)
                .filter(TI.state.in_(State.unfinished))