### 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)

from airflow.timetables.simple import DatasetTriggeredTimetable
from airflow.utils import timezone
from airflow.utils.event_scheduler import EventScheduler
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.retries import MAX_DB_RETRIES, retry_db_transaction, run_with_db_retries
from airflow.utils.session import NEW_SESSION, create_session, provide_session
from airflow.utils.sqlalchemy import (
    CommitProhibitorGuard,
    is_lock_not_available_error,
    prohibit_commit,
    skip_locked,
    tuple_in_condition,
    with_row_locks,
)
from airflow.utils.state import DagRunState, State, TaskInstanceState
from airflow.utils.types import DagRunType
if TYPE_CHECKING:
    from types import FrameType
    from airflow.dag_processing.manager import DagFileProcessorAgent
TI = TaskInstance
DR = DagRun
DM = DagModel
def _is_parent_process() -> bool:
    """
    Whether this is a parent process.
    Return True if the current process is the parent process.
    False if the current process is a child process started by multiprocessing.
    """
    return multiprocessing.current_process().name == "MainProcess"
class SchedulerJobRunner(BaseJobRunner, LoggingMixin):