### 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)

)
            for task_instance in unfinished_task_instances:
                task_instance.state = TaskInstanceState.SKIPPED
                session.merge(task_instance)
            session.flush()
            self.log.info("Run %s of %s has timed-out", dag_run.run_id, dag_run.dag_id)
            active_runs = dag.get_num_active_runs(only_running=False, session=session)
            
            if self._should_update_dag_next_dagruns(dag, dag_model, active_runs):
                dag_model.calculate_dagrun_date_fields(dag, dag.get_run_data_interval(dag_run))
            callback_to_execute = DagCallbackRequest(
                full_filepath=dag.fileloc,
                dag_id=dag.dag_id,
                run_id=dag_run.run_id,
                is_failure_callback=True,
                processor_subdir=dag_model.processor_subdir,
                msg="timed_out",
            )
            dag_run.notify_dagrun_state_changed()
            duration = dag_run.end_date - dag_run.start_date
            Stats.timing(f"dagrun.duration.failed.{dag_run.dag_id}", duration)
            Stats.timing("dagrun.duration.failed", duration, tags={"dag_id": dag_run.dag_id})
            return callback_to_execute
        if dag_run.execution_date > timezone.utcnow() and not dag.allow_future_exec_dates:
            self.log.error("Execution date is in future: %s", dag_run.execution_date)
            return callback
        if not self._verify_integrity_if_dag_changed(dag_run=dag_run, session=session):
            self.log.warning("The DAG disappeared before verifying integrity: %s. Skipping.", dag_run.dag_id)
            return callback