[feature] Add counter for number of changes detected

Add counter for number of changes detected , handy for debugging

No file, test, or entry point is named. Start by locating the change-detection code and tracing where changes are identified, then clarify what events the counter should include and where it should be exposed. Done means the counter reports the agreed number of detected changes and has coverage for the expected behavior.

Written by the indexing model from the issue text.