If you want to run it from the client, then set it up as a Business Process and set rules regarding who is allowed to run it and what conditions must be met for it to run.

Next, set it up to require a starting entry and when you run it, the selected folder will be the starting entry.

In the workflow, use a Find Entries activity that will grab all the contents of your starting entry (the folder you selected before running the process).

Then, set up a For Each Entry loop to go through all of the results.

Inside the For Each put a Conditional decision

If Entry Type is Document > Update Metadata

If Entry Type is Folder > Invoke Workflow with the "Current Entry" as the Starting Entry and set it to Wait for the task to complete.

What this will do is loop through every item in your starting folder. If you encounter a document it will update the metadata.

However, if it encounters a folder, it will kick off the same process for that folder and wait for it to finish, and so on and on like a recursive loop.

You could use a Search Repository activity, but those have a lot of overhead and it will all run in one gigantic process. The nice thing about the recursive workflow approach is that you get a separate instance for each subfolder so it will run more efficiently.