### 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)

try:
                    num_failed = (
                        session.query(Job)
                        .filter(
                            Job.job_type == "SchedulerJob",
                            Job.state == State.RUNNING,
                            Job.latest_heartbeat < (timezone.utcnow() - timedelta(seconds=timeout)),
                        )
                        .update({"state": State.FAILED})
                    )
                    if num_failed:
                        self.log.info("Marked %d SchedulerJob instances as failed", num_failed)
                        Stats.incr(self.__class__.__name__.lower() + "_end", num_failed)
                    resettable_states = [TaskInstanceState.QUEUED, TaskInstanceState.RUNNING]
                    query = (
                        session.query(TI)
                        .filter(TI.state.in_(resettable_states))
                        
                        
                        
                        
                        .outerjoin(TI.queued_by_job)
                        .filter(or_(TI.queued_by_job_id.is_(None), Job.state != State.RUNNING))
                        .join(TI.dag_run)
                        .filter(
                            DagRun.run_type != DagRunType.BACKFILL_JOB,
                            DagRun.state == State.RUNNING,
                        )
                        .options(load_only(TI.dag_id, TI.task_id, TI.run_id))
                    )