### 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)

"""
        Find zombie task instances, which are tasks haven't heartbeated for too long
        or have a no-longer-running LocalTaskJob, and create a TaskCallbackRequest
        to be handled by the DAG processor.
        """
        from airflow.jobs.job import Job
        self.log.debug("Finding 'running' jobs without a recent heartbeat")
        limit_dttm = timezone.utcnow() - timedelta(seconds=self._zombie_threshold_secs)
        with create_session() as session:
            zombies: list[tuple[TI, str, str]] = (
                session.query(TI, DM.fileloc, DM.processor_subdir)
                .with_hint(TI, "USE INDEX (ti_state)", dialect_name="mysql")
                .join(Job, TI.job_id == Job.id)
                .join(DM, TI.dag_id == DM.dag_id)
                .filter(TI.state == TaskInstanceState.RUNNING)
                .filter(
                    or_(
                        Job.state != State.RUNNING,
                        Job.latest_heartbeat < limit_dttm,
                    )
                )
                .filter(Job.job_type == "LocalTaskJob")
                .filter(TI.queued_by_job_id == self.job.id)
                .all()
            )
        if zombies: