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 中没有指明任何 repository 文件或测试。首先检查当前 bundled decorator 实现和链接的 Fedora unbundling patch，然后确定项目将依赖外部 package，还是保留更新后的 bundled 版本。选定的依赖策略能够在当前 Python 版本上正常工作，并且其兼容性由现有测试套件覆盖，即视为完成。