chore: add missing generated artifact entries to .gitignore

Add .phpunit.cache/, coverage/, and phpmetrics/ entries to prevent
generated test and quality tool artifacts from being tracked.

Author: rubenvdlinde

.gitignore

@@ -16,6 +16,9 @@
 /coverage/
 /coverage-frontend/
 
+.phpunit.cache/
+phpmetrics/
+
 # Data files
 /data/
 *.csv