fix: Python3.8 type compatibility

[bilbobx182]

What does this do?

There's currently a breaking problem with python 3.8 support.

    from prometheus_fastapi_instrumentator import Instrumentator
  File "/pythonlibs/prometheus_fastapi_instrumentator/__init__.py", line 1, in <module>
    from .instrumentation import PrometheusFastApiInstrumentator
  File "/pythonlibs/prometheus_fastapi_instrumentator/instrumentation.py", line 21, in <module>
    from prometheus_fastapi_instrumentator import metrics
  File "/pythonlibs/prometheus_fastapi_instrumentator/metrics.py", line 601, in <module>
    def _map_label_name_value(label_name: tuple) -> list[str]:
TypeError: 'type' object is not subscriptable
Process SpawnProcess-4:

Why do we need it?

To support python3.8 we need to use the typing module to support it. It's already in use elsewhere in the file, however when this feature was added it used more modern python types.
This is a trivial change of changing it to use the typing module.

Who is this for?

Anyone who wants to use the library.

Linked issues

Stems from these changes
#326