**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
self, dagbag: DagBag, callback_requests: List[CallbackRequest], session: Session = None
    ) -> None:
        """
        Execute on failure callbacks. These objects can come from SchedulerJob or from
        DagFileProcessorManager.
        :param dagbag: Dag Bag of dags
        :param callback_requests: failure callbacks to execute
        :type callback_requests: List[airflow.utils.callback_requests.CallbackRequest]
        :param session: DB session.
        """
        for request in callback_requests:
            try:
                if isinstance(request, TaskCallbackRequest):
                    self._execute_task_callbacks(dagbag, request)
                elif isinstance(request, SlaCallbackRequest):
                    self.manage_slas(dagbag.dags.get(request.dag_id))
                elif isinstance(request, DagCallbackRequest):
                    self._execute_dag_callbacks(dagbag, request, session)
            except Exception:  
                self.log.exception(
                    "Error executing %s callback for file: %s",
                    request.__class__.__name__,
                    request.full_filepath,
                )
        session.commit()
    @provide_session
    def _execute_dag_callbacks(self, dagbag: DagBag, request: DagCallbackRequest, session: Session):