**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

tests/serialization/test_dag_serialization.py
"""OperatorLink not registered via Plugins nor a built-in OperatorLink"""
            name = 'My Link'
            def get_link(self, operator, dttm):
                return 'https://www.google.com'
        class MyOperator(BaseOperator):
            """Just a DummyOperator using above defined Extra Operator Link"""
            operator_extra_links = [TaskStateLink()]
            def execute(self, context):
                pass
        with DAG(dag_id='simple_dag', start_date=datetime(2019, 8, 1)) as dag:
            MyOperator(task_id='blah')
        serialized_dag = SerializedDAG.to_dict(dag)
        with self.assertLogs("airflow.serialization.serialized_objects", level="ERROR") as log_output:
            SerializedDAG.from_dict(serialized_dag)
            received_logs = log_output.output[0]
            expected_err_msg = (
                "Operator Link class 'tests.serialization.test_dag_serialization.TaskStateLink' "
                "not registered"
            )
            assert expected_err_msg in received_logs
    def test_extra_serialized_field_and_multiple_operator_links(self):