**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,
        subdir: str = settings.DAGS_FOLDER,
        num_runs: int = conf.getint('scheduler', 'num_runs'),
        num_times_parse_dags: int = -1,
        processor_poll_interval: float = conf.getfloat('scheduler', 'processor_poll_interval'),
        do_pickle: bool = False,
        log: Any = None,
        *args,
        **kwargs,
    ):
        self.subdir = subdir
        self.num_runs = num_runs
        
        
        
        self.num_times_parse_dags = num_times_parse_dags
        self._processor_poll_interval = processor_poll_interval
        self.do_pickle = do_pickle
        super().__init__(*args, **kwargs)
        if log:
            self._log = log
        
        sql_conn: str = conf.get('core', 'sql_alchemy_conn').lower()
        self.using_sqlite = sql_conn.startswith('sqlite')
        self.using_mysql = sql_conn.startswith('mysql')
        self.max_tis_per_query: int = conf.getint('scheduler', 'max_tis_per_query')
        self.processor_agent: Optional[DagFileProcessorAgent] = None
        self.dagbag = DagBag(dag_folder=self.subdir, read_dags_from_db=True, load_op_links=False)
    def register_signals(self) -> None: