**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/serialization/serialized_objects.py
"""Serializes a DAG into a JSON object."""
        try:
            serialize_dag = cls.serialize_to_json(dag, cls._decorated_fields)
            serialize_dag["tasks"] = [cls._serialize(task) for _, task in dag.task_dict.items()]
            serialize_dag['_task_group'] = SerializedTaskGroup.serialize_task_group(dag.task_group)
            
            if dag.has_on_success_callback:
                serialize_dag['has_on_success_callback'] = True
            if dag.has_on_failure_callback:
                serialize_dag['has_on_failure_callback'] = True
            return serialize_dag
        except SerializationError:
            raise
        except Exception:
            raise SerializationError(f'Failed to serialize dag {dag.dag_id!r}')
    @classmethod
    def deserialize_dag(cls, encoded_dag: Dict[str, Any]) -> 'SerializedDAG':