I originally reported this issue to Rails but they said that they don't do bug fixes on Rails v6.1x anymore and since this is more of a Ruby bug (or perhaps not thinking through the removal of the Dir ...

Ruby Method Dispatch In order to do a method call in Ruby, a runtime simply looks at the target object’s class hierarchy, searches for the method from bottom to top, and upon finding it performs the ...

First of all you need to initialize the gem using Method::Tracer.configure method. You need to supply an instance of a tracer, and an active span provider - a proc which returns a current active span.