feat: expose __version__ in GAPIC clients

[parthea]

This PR adds a version module to allow users to query the version of the GAPIC.

(py39) partheniou@partheniou-vm-2:~/git/python-monitoring$ python3
Python 3.9.10 (main, Feb 22 2022, 13:54:07) 
[GCC 11.2.0] on linux
Type "help", "copyright", "credits" or "license" for more information.
>>> from google.cloud.monitoring import version
>>> version.__version__
'2.9.1'

fix: use version.py instead of pkg_resources.get_distribution fix: drop dependency on setuptools