**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
cls, object_to_serialize: Union[BaseOperator, DAG], decorated_fields: Set
    ) -> Dict[str, Any]:
        """Serializes an object to json"""
        serialized_object: Dict[str, Any] = {}
        keys_to_serialize = object_to_serialize.get_serialized_fields()
        for key in keys_to_serialize:
            
            value = getattr(object_to_serialize, key, None)
            if cls._is_excluded(value, key, object_to_serialize):
                continue
            if key in decorated_fields:
                serialized_object[key] = cls._serialize(value)
            else:
                value = cls._serialize(value)
                if isinstance(value, dict) and "__type" in value:
                    value = value["__var"]
                serialized_object[key] = value
        return serialized_object
    
    @classmethod
    def _serialize(cls, var: Any) -> Any: