### 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)

"""
        Detects datasets that are no longer referenced in any DAG schedule parameters or task outlets and
        sets the dataset is_orphaned flag to True
        """
        orphaned_dataset_query = (
            session.query(DatasetModel)
            .join(
                DagScheduleDatasetReference,
                isouter=True,
            )
            .join(
                TaskOutletDatasetReference,
                isouter=True,
            )
            
            
            .group_by(DatasetModel if session.get_bind().dialect.name == "mssql" else DatasetModel.id)
            .having(
                and_(
                    func.count(DagScheduleDatasetReference.dag_id) == 0,
                    func.count(TaskOutletDatasetReference.dag_id) == 0,
                )
            )
        )
        updated_count = sum(self._set_orphaned(dataset) for dataset in orphaned_dataset_query)
        Stats.gauge("dataset.orphaned", updated_count)