### 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 dag:
                self.log.error("DAG '%s' not found in serialized_dag table", dag_model.dag_id)
                continue
            dag_hash = self.dagbag.dags_hash.get(dag.dag_id)
            data_interval = dag.get_next_data_interval(dag_model)
            
            
            
            
            
            
            
            
            if (dag.dag_id, dag_model.next_dagrun) not in existing_dagruns:
                dag.create_dagrun(
                    run_type=DagRunType.SCHEDULED,
                    execution_date=dag_model.next_dagrun,
                    state=DagRunState.QUEUED,
                    data_interval=data_interval,
                    external_trigger=False,
                    session=session,
                    dag_hash=dag_hash,
                    creating_job_id=self.job.id,
                )
                active_runs_of_dags[dag.dag_id] += 1
            if self._should_update_dag_next_dagruns(dag, dag_model, active_runs_of_dags[dag.dag_id]):
                dag_model.calculate_dagrun_date_fields(dag, data_interval)
        
        
    def _create_dag_runs_dataset_triggered(