### 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)

ti.task_id,
                ti.run_id,
                ti.map_index,
                ti.start_date,
                ti.end_date,
                ti.duration,
                ti.state,
                state,
                try_number,
                ti.max_tries,
                ti.job_id,
                ti.pool,
                ti.queue,
                ti.priority_weight,
                ti.operator,
                ti.queued_dttm,
                ti.queued_by_job_id,
                ti.pid,
            )
            
            
            
            
            
            
            
            
            ti_queued = ti.try_number == buffer_key.try_number and ti.state == TaskInstanceState.QUEUED
            ti_requeued = ti.queued_by_job_id != self.job.id or self.job.executor.has_task(ti)
            if ti_queued and not ti_requeued: