### 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)

"""
        Looks at all tasks that are in the "deferred" state and whose trigger
        or execution timeout has passed, so they can be marked as failed.
        """
        num_timed_out_tasks = (
            session.query(TaskInstance)
            .filter(
                TaskInstance.state == TaskInstanceState.DEFERRED,
                TaskInstance.trigger_timeout < timezone.utcnow(),
            )
            .update(
                
                
                {
                    "state": TaskInstanceState.SCHEDULED,
                    "next_method": "__fail__",
                    "next_kwargs": {"error": "Trigger/execution timeout"},
                    "trigger_id": None,
                }
            )
        )
        if num_timed_out_tasks:
            self.log.info("Timed out %i deferred tasks without fired triggers", num_timed_out_tasks)
    def _find_zombies(self) -> None: