Hi. Some ideas:
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XWiki architecture: https://www.xwiki.org/xwiki/bin/view/Documentation/DevGuide/Architecture/
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Your best bet if you want your metric to be “accessed often” is going to be to find out the extensions that are the most used. In XWiki a feature is an extension (not exactly but that’s what we would like it to be). The problem is that users install XWiki Standard which is a bundle of extensions and thus all these extensions (features) count as being installed. So you won’t know which ones are being used the most. You can only know which additional (non core) extensions are being installed the most.
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Active installs of extensions: https://extensions.xwiki.org/ (in the table there’s a column)
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Active usage of extensions is also available in Kibana (we collect the following info: https://extensions.xwiki.org/xwiki/bin/view/Extension/Active%20Installs%20Client%20API). See also https://extensions.xwiki.org/xwiki/bin/view/Extension/Active%20Installs%20Server%20Application . Kibana at Kibana 3
Selenium is not tracing tool. How do you plan to use it for tracing? 
More generally this is a topic that would be interesting to us: knowing which features of XWiki are being used the most. For that we would need to implement something similar to Active Installations but for usage of features.
Good luck.