### 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)

self,
        job: Job,
        subdir: str = settings.DAGS_FOLDER,
        num_runs: int = conf.getint("scheduler", "num_runs"),
        num_times_parse_dags: int = -1,
        scheduler_idle_sleep_time: float = conf.getfloat("scheduler", "scheduler_idle_sleep_time"),
        do_pickle: bool = False,
        log: logging.Logger | None = None,
        processor_poll_interval: float | None = None,
    ):
        super().__init__()
        if job.job_type and job.job_type != self.job_type:
            raise Exception(
                f"The job is already assigned a different job_type: {job.job_type}."
                f"This is a bug and should be reported."
            )
        self.job = job
        self.job.job_type = self.job_type
        self.subdir = subdir
        self.num_runs = num_runs