If you have a DAG with `max_active_runs=1` and it is running some tasks, and then you manually trigger another run of this DAG, both runs end up not scheduling anything and you see this message over and over in the logs:

```
[2020-10-16 14:00:34,118]  2437572 {{airflow.jobs.scheduler_job.SchedulerJob scheduler_job.py:1657}} INFO - DAG scenario1_case2_01_1 already has 1 active runs, not queuing any more tasks
```

A longer term fix for this would be to introduce a "queued" state for DagRuns, and then when manually creating dag runs set it to queued, and only have the scheduler set DagRuns to running.

That may or may not be less work than fixing the current mechanism.
airflow/jobs/scheduler_job.py
self,
        dag_run: DagRun,
        callback: Optional[DagCallbackRequest] = None
    ):
        if not self.processor_agent:
            raise ValueError("Processor agent is not started.")
        dag = dag_run.get_dag()
        self._send_sla_callbacks_to_processor(dag)
        if callback:
            self.processor_agent.send_callback_to_execute(callback)
    def _send_sla_callbacks_to_processor(self, dag: DAG):
        """Sends SLA Callbacks to DagFileProcessor if tasks have SLAs set and check_slas=True"""
        if not settings.CHECK_SLAS:
            return
        if not any(isinstance(ti.sla, timedelta) for ti in dag.tasks):
            self.log.debug("Skipping SLA check for %s because no tasks in DAG have SLAs", dag)
            return
        if not self.processor_agent:
            raise ValueError("Processor agent is not started.")
        self.processor_agent.send_sla_callback_request_to_execute(
            full_filepath=dag.fileloc,
            dag_id=dag.dag_id
        )
    @provide_session
    def _emit_pool_metrics(self, session: Session = None) -> None: