Improve search for event extras

[wlach]

Sometimes the event extras field has important information that would show a metric to be relevant where you wouldn't expect it to be just from the name or description.

For example:

https://dictionary.telemetry.mozilla.org/apps/fenix/metrics/events_browser_menu_action

has the extra item which is:

A string containing the name of the item the user tapped. These items include: add_to_homescreen, add_to_top_sites, addons_manager, back, bookmark, bookmarks, desktop_view_off, desktop_view_on, downloads, find_in_page, forward, help, history, library, new_private_tab, new_tab, open_in_app, open_in_fenix, quit, reader_mode_appearance, reader_mode_off, reader_mode_on, reload, save_to_collection, set_default_browser, settings, share, stop, sync_account, and sync_tabs.

One wrinkle which may make this feature less useful: the extras are not exposed in Looker's event explores by default.

Also, you could make the argument that we could add tags to make these more discoverable (though this event would have a lot of tags!)