how to extend watch function

i want to add some self-define func into watch functions, but can't find extension point

Start by locating Consul's watch-function implementation and any documentation describing extension points. The issue does not name a package, entry point, desired custom behavior, or acceptance criteria, so first clarify what self-defined functions should receive and how they should be registered before implementation can be considered complete.

Written by the indexing model from the issue text.