Log Message:
py-prometheus_client: update to 0.21.0.

What's Changed

[CHANGE] Reject invalid (not GET or OPTION) HTTP methods. #1019
[ENHANCEMENT] Allow writing metrics when holding a lock for the metric in the \ 
same thread. #1014
[BUGFIX] Check for and error on None label values. #1012
[BUGFIX] Fix timestamp comparison. #1038