### 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)

existing_dagruns: set[tuple[str, timezone.DateTime]] = set(
            session.query(DagRun.dag_id, DagRun.execution_date).filter(
                tuple_in_condition((DagRun.dag_id, DagRun.execution_date), exec_dates.items())
            )
        )
        for dag_model in dag_models:
            dag = self.dagbag.get_dag(dag_model.dag_id, session=session)
            if not dag:
                self.log.error("DAG '%s' not found in serialized_dag table", dag_model.dag_id)
                continue
            if not isinstance(dag.timetable, DatasetTriggeredTimetable):
                self.log.error(
                    "DAG '%s' was dataset-scheduled, but didn't have a DatasetTriggeredTimetable!",
                    dag_model.dag_id,
                )
                continue
            dag_hash = self.dagbag.dags_hash.get(dag.dag_id)
            
            
            
            
            
            
            
            
            exec_date = exec_dates[dag.dag_id]
            if (dag.dag_id, exec_date) not in existing_dagruns:
                previous_dag_run = (
                    session.query(DagRun)
                    .filter(