FileBrowser.DrivePins - recordUserAction() every time a file is pinned
FileBrowser.DriveUnpins - recordUserAction() every time a file is unpinned
FileBrowser.DrivePinFileCount - recordMediumCount(total number of pinned files) every time a file is pinned/unpinned
FileBrowser.DrivePinFileSizeInMegabytes - recordCount(total size of pinned files) every time a file is pinned/unpinned

For an idea of how to implement, see the existing FileBrowser.DownloadsCount UMA (which records the number of files and directories in Downloads) and FileBrowser.DirectoryScan (which records the time to scan a directory).

The available metrics methods are in metrics_base.js: https://cs.chromium.org/chromium/src/ui/file_manager/file_manager/common/js/metrics_base.js?type=cs&sq=package:chromium