### 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._emit_pool_metrics,
        )
        timers.call_regular_interval(
            conf.getfloat("scheduler", "zombie_detection_interval", fallback=10.0),
            self._find_zombies,
        )
        timers.call_regular_interval(60.0, self._update_dag_run_state_for_paused_dags)
        timers.call_regular_interval(
            conf.getfloat("scheduler", "parsing_cleanup_interval"),
            self._orphan_unreferenced_datasets,
        )
        if self._standalone_dag_processor:
            timers.call_regular_interval(
                conf.getfloat("scheduler", "parsing_cleanup_interval"),
                self._cleanup_stale_dags,
            )
        for loop_count in itertools.count(start=1):
            with Stats.timer("scheduler.scheduler_loop_duration") as timer:
                if self.using_sqlite and self.processor_agent:
                    self.processor_agent.run_single_parsing_loop()
                    
                    
                    self.log.debug("Waiting for processors to finish since we're using sqlite")
                    self.processor_agent.wait_until_finished()
                with create_session() as session:
                    num_queued_tis = self._do_scheduling(session)
                    self.job.executor.heartbeat()
                    session.expunge_all()
                    num_finished_events = self._process_executor_events(session=session)
                if self.processor_agent: