### 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)

)
                dag_run = dag.create_dagrun(
                    run_id=run_id,
                    run_type=DagRunType.DATASET_TRIGGERED,
                    execution_date=exec_date,
                    data_interval=data_interval,
                    state=DagRunState.QUEUED,
                    external_trigger=False,
                    session=session,
                    dag_hash=dag_hash,
                    creating_job_id=self.job.id,
                )
                Stats.incr("dataset.triggered_dagruns")
                dag_run.consumed_dataset_events.extend(dataset_events)
                session.query(DatasetDagRunQueue).filter(
                    DatasetDagRunQueue.target_dag_id == dag_run.dag_id
                ).delete()
    def _should_update_dag_next_dagruns(self, dag, dag_model: DagModel, total_active_runs: int) -> bool:
        """Check if the dag's next_dagruns_create_after should be updated."""
        if total_active_runs >= dag.max_active_runs:
            self.log.info(
                "DAG %s is at (or above) max_active_runs (%d of %d), not creating any more runs",
                dag_model.dag_id,
                total_active_runs,
                dag.max_active_runs,
            )
            dag_model.next_dagrun_create_after = None
            return False
        return True
    def _start_queued_dagruns(self, session: Session) -> None: