Unify Callback Function Naming

[MrAlias]

From recent feedback:

Currently, there isn’t a convention in naming for callback-like functions. GetHookFunc, BatchObserverCallback. This gives users an impression there is a specific reason they are named differently and may have differences in behavior although they don’t.
We can pick one for consistency and developer productivity. *Callback is used less in Go, *Func or *Handler would be a good fit.

[dengliming]

@MrAlias Hello, I'd love to take this as first contribution for this repo. I suggest rename *Callback with *Handler. WDYT?

[MrAlias]

Thanks for the help! It's much appreciated 🎉. I'll assign this issue to you.

@MrAlias Hello, I'd love to take this as first contribution for this repo. I suggest rename *Callback with *Handler. WDYT?

I'd prefer to go with *Func instead of *Handler. This is based on the fact that we have an ErrorHandler that is an interface not fitting the callback-like function form. Using a *Func suffix would already have some implementations, be shorter, fit a bit closer to the functional nature of the call (this is subjective for sure), and wouldn't conflict with the existing instance of the ErrorHandler.

Additionally, our existing Bound* instruments used to be called Handlers*. This was changed so there wouldn't be conflict there, but there are still some lurking remnants (maybe just this so not the biggest deal 🤷) of this naming that might confuse if we went with *Handler.

[MrAlias]

resolved in #1061