**Apache Airflow version**: 2.0.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:

I have the following DAG:

```py
from datetime import datetime, timedelta

from airflow import DAG
from airflow.operators.bash_operator import BashOperator

with DAG(
    dag_id="sla_trigger",
    schedule_interval="* * * * *",
    start_date=datetime(2020, 2, 3),
) as dag:
    BashOperator(
        task_id="bash_task",
        bash_command="sleep 30",
        sla=timedelta(seconds=2),
    )

```

And in my understanding this dag should result in SLA miss every time it is triggered (every minute). However, after few minutes of running I don't see any SLA misses...

**What you expected to happen**:

I expect to see SLA if task takes longer than expected.

**How to reproduce it**:

Use the dag from above.

**Anything else we need to know**:

N/A

airflow/jobs/scheduler_job.py
dag_ids=[],
            pickle_dags=pickle_dags,
            async_mode=async_mode,
        )
        try:
            self.executor.job_id = self.id
            self.executor.start()
            self.register_signals()
            self.processor_agent.start()
            execute_start_time = timezone.utcnow()
            self._run_scheduler_loop()
            
            self.processor_agent.terminate()
            
            
            
            if self.processor_agent.all_files_processed:
                self.log.info(
                    "Deactivating DAGs that haven't been touched since %s", execute_start_time.isoformat()
                )
                models.DAG.deactivate_stale_dags(execute_start_time)
            self.executor.end()
            settings.Session.remove()  
        except Exception:  
            self.log.exception("Exception when executing SchedulerJob._run_scheduler_loop")
        finally:
            self.processor_agent.end()
            self.log.info("Exited execute loop")
    @staticmethod
    def _create_dag_file_processor(