Add Support for Non-Default Google Cloud Universes via a flag

[matgou]

Description:

This issue tracks the implementation of a new feature that allows users to specify a non-default Google Cloud universe when using the stackdriver_exporter.

Problem:

Currently, the stackdriver_exporter assumes the default googleapis.com universe for all interactions with Google Cloud services. This limits its usability for users who operate within different Google Cloud universes (e.g., s3nsapis.fr).

Solution:

This feature introduces a new command-line flag, (google.universe-domain ?), which enables users to explicitly define the Google Cloud universe they want to interact with. The default value for this flag is googleapis.com, ensuring backward compatibility.