Add a cardinality question to the PR template

Reviewing a pull request involves assesing in which way the introduced change
affects metrics cardinality. To ease the burder on the reviewer and to make
contributors aware of the constraint, this commit extends the pull request template
with a question about the effect on metrics cardinality.

.github/PULL_REQUEST_TEMPLATE.md

@@ -7,6 +7,7 @@
 
 **What this PR does / why we need it**:
 
+**How does this change affect the cardinality of KSM**: *(increases, decreases or does not change cardinality)*
+
 **Which issue(s) this PR fixes** *(optional, in `fixes #<issue number>(, fixes #<issue_number>, ...)` format, will close the issue(s) when PR gets merged)*:
 Fixes #
-