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.

issue では、リポジトリのファイルもテストも指定されていません。まず現在の bundled decorator 実装と、リンクされている Fedora の unbundling patch を確認し、その後、プロジェクトが外部パッケージに依存するのか、更新された bundled 版を維持するのかを判断してください。選択した依存関係の方針が現在の Python バージョンで動作し、その互換性が既存のテストスイートでカバーされていれば完了です。