Log Message:
py-prometheus_client: update to 0.18.0.

[CHANGE] Remove support for Python versions < 3.8. #936
[FEATURE] Add mostrecent aggregation to Gauge. #967
[ENHANCEMENT] Typing improvements. #935, #970
[ENHANCEMENT] Allow enabling or disabling _created metrics from code. #973
[BUGFIX] Change #!/usr/bin/python to #!/usr/bin/env python in remaining places. #945