add metric that counts times page hidden

[ansis]

Adds a metric to count the number of times page visibility has changed to "hidden". This supports performance metrics by identifying situations where long times could be the result of switching away from the page.

Launch Checklist

• [x] briefly describe the changes in this PR
• [ ] write tests for all new functionality
• [x] manually test the debug page
• [x] apply changelog label ('bug', 'feature', 'docs', etc) or use the label 'skip changelog'