**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
assert set(simple_task.extra_links) == {
            'BigQuery Console #1',
            'BigQuery Console #2',
            'airflow',
            'github',
            'google',
        }
        ti = TaskInstance(task=simple_task, execution_date=test_date)
        ti.xcom_push('search_query', ["dummy_value_1", "dummy_value_2"])
        
        custom_inbuilt_link = simple_task.get_extra_links(test_date, "BigQuery Console #1")
        assert 'https://console.cloud.google.com/bigquery?j=dummy_value_1' == custom_inbuilt_link
        
        custom_inbuilt_link = simple_task.get_extra_links(test_date, "BigQuery Console #2")
        assert 'https://console.cloud.google.com/bigquery?j=dummy_value_2' == custom_inbuilt_link
        
        google_link_from_plugin = simple_task.get_extra_links(test_date, GoogleLink.name)
        assert "https://www.google.com" == google_link_from_plugin
    class ClassWithCustomAttributes:
        """
        Class for testing purpose: allows to create objects with custom attributes in one single statement.
        """
        def __init__(self, **kwargs):
            for key, value in kwargs.items():
                setattr(self, key, value)
        def __str__(self):
            return f"{self.__class__.__name__}({str(self.__dict__)})"
        def __repr__(self):
            return self.__str__()
        def __eq__(self, other):