**Apache Airflow version**: 2.1.0

**What happened**:

Using a Label infront of a TaskGroup breaks task dependencies, as tasks before the TaskGroup will now point to the last task in the TaskGroup.

**What you expected to happen**:

Task dependencies with or without Labels should be the same (and preferably the Label should be visible outside of the TaskGroup).
Behavior without a Label is as follows:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

DummyOperator(task_id="a") >> tg1
```
![without Taskgroup](https://user-images.githubusercontent.com/35238779/128502690-58b31577-91d7-4b5c-b348-4779c9286958.png)

**How to reproduce it**:

```python
with TaskGroup("tg1", tooltip="Tasks related to task group") as tg1:
        DummyOperator(task_id="b") >> DummyOperator(task_id="c")

    DummyOperator(task_id="a") >> Label("testlabel") >> tg1
```
![with TaskGroup](https://user-images.githubusercontent.com/35238779/128502758-5252b7e7-7c30-424a-ac3a-3f9a99b0a805.png)


"""Number of instances a task in this group should be mapped to at run time.
        This considers both literal and non-literal mapped arguments, and the
        result is therefore available when all depended tasks have finished. The
        return value should be identical to ``parse_time_mapped_ti_count`` if
        all mapped arguments are literal.
        If this group is inside mapped task groups, all the nested counts are
        multiplied and accounted.
        :meta private:
        :raise NotFullyPopulated: If upstream tasks are not all complete yet.
        :return: Total number of mapped TIs this task should have.
        """
        groups = self.iter_mapped_task_groups()
        return functools.reduce(
            operator.mul,
            (g._expand_input.get_total_map_length(run_id, session=session) for g in groups),
        )
class TaskGroupContext: