Unbundle the `decorator` dependency, or at least upgrade it

Hi,

In #91 years ago, it was decided to add the decorator to fix a problem. Back then choices was made to not add a new dependency, but really bundle the only decorar file needed. Distributions (in my case Fedora) usually prefer to rely on the actual package than bundled dependencies. Fedora does apply a patch to unbundle decorator, maybe other distro do that too.

Do we still want to avoid dependencies in 2025 ? I have seen other discussions where it was discussed adding a protobuf dependency, which pulls many more dependencies, so isn't ok now to depend on decorator ?

If not, would it make sense at least to upgrade it ? By doing so, you hit the issue that decorator.getargspec has been removed years ago. In python 3 there is now inspect.getfullargspec which is not strictly equivalent but does provide a similar feature. Fedora uses a minimal patch here: https://src.fedoraproject.org/rpms/python-prometheus_client/blob/rawhide/f/0001-Remove-the-bundled-decorator-package.patch#_506 but tries to avoid adding much code, most likely the "real" change is more verbose.

In der Issue wird keine Repository-Datei und kein Test genannt. Beginne damit, die aktuelle gebündelte Decorator-Implementierung und den verlinkten Fedora-Unbundling-Patch zu prüfen, und ermittle dann, ob das Projekt von dem externen Paket abhängen oder eine aktualisierte gebündelte Version beibehalten wird. Als abgeschlossen gilt die Aufgabe, wenn die gewählte Abhängigkeitsstrategie mit den aktuellen Python-Versionen funktioniert und ihre Kompatibilität durch die bestehende Testsuite abgedeckt ist.