**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
value = getattr(op, template_field, None)
                if not cls._is_excluded(value, template_field, op):
                    serialize_op[template_field] = serialize_template_field(value)
        return serialize_op
    @classmethod
    def deserialize_operator(cls, encoded_op: Dict[str, Any]) -> BaseOperator:
        """Deserializes an operator from a JSON object."""
        op = SerializedBaseOperator(task_id=encoded_op['task_id'])
        if "label" not in encoded_op:
            
            encoded_op["label"] = encoded_op["task_id"]
        
        op_extra_links_from_plugin = {}
        
        if cls._load_operator_extra_links:  
            from airflow import plugins_manager
            plugins_manager.initialize_extra_operators_links_plugins()
            if plugins_manager.operator_extra_links is None:
                raise AirflowException("Can not load plugins")
            for ope in plugins_manager.operator_extra_links:
                for operator in ope.operators:
                    if (
                        operator.__name__ == encoded_op["_task_type"]
                        and operator.__module__ == encoded_op["_task_module"]
                    ):
                        op_extra_links_from_plugin.update({ope.name: ope})