Add file tree browser instrumentation metrics

What does this MR do and why?

This is part of a bigger effort to implement a file tree browser (FTB) in the repository. All changes are behind repository_file_tree_browser FF.

Specifically, this MR adds 3 metrics for the events added in Add analytics instrumentation for file tree bro... (!203015 - merged).

References

[instrumentation plan] - file tree browser (#562718 - closed)

MR acceptance checklist

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Related to #562718 (closed)