### 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)

if not _is_parent_process():
            
            return
        try:
            sig_name = signal.Signals(signum).name
        except Exception:
            sig_name = str(signum)
        self.log.info("%s\n%s received, printing debug\n%s", "-" * 80, sig_name, "-" * 80)
        self.job.executor.debug_dump()
        self.log.info("-" * 80)
    def is_alive(self, grace_multiplier: float | None = None) -> bool:
        """
        Whether the SchedulerJob is alive.
        We define alive as in a state of running and a heartbeat within the
        threshold defined in the ``scheduler_health_check_threshold`` config
        setting.
        ``grace_multiplier`` is accepted for compatibility with the parent class.
        """
        if grace_multiplier is not None:
            
            return self.job.is_alive(grace_multiplier=grace_multiplier)
        scheduler_health_check_threshold: int = conf.getint("scheduler", "scheduler_health_check_threshold")
        return (
            self.job.state == State.RUNNING
            and (timezone.utcnow() - self.job.latest_heartbeat).total_seconds()
            < scheduler_health_check_threshold
        )
    def __get_concurrency_maps(