There is no way to do this (in Confluence either - what you've seen for Confluence is a way to find unused macros that may well be provided by addons, but not the full range of all add-ons). The problem is that an add-on can do all sorts of things, and the way they're implemented makes an across-the-board search impossible.
As an example, I often use the Script Runner to provide scripted fields and post functions. They have nothing in common to search for. I can find fields by reading the list of fields, or looking in customfield in the database. To find post functions, you have to read workflows one way or another. I can't even search for "is script runner used" in one question, let alone any other add-on.
I'm afraid you've got three basic options:
Connect with like-minded Atlassian users at free events near you!Find a group
Connect with like-minded Atlassian users at free events near you!
Unfortunately there are no AUG chapters near you at the moment.Start an AUG
We're bringing product updates and pro tips on teamwork to ten cities around the world.Save your spot