Each processor might end up in multiple processor groups.
Every group has its own `Close` that calls `Close` on each
processor of that group which leads to multiple `Close` calls
on the same processor.

If the `SafeProcessor` wrapper was not used,
the processor would have to handle multiple `Close` calls with
adding `sync.Once` in its `Close` function.

We make it easer for processor developers and take care of it
in the processor registry instead.

Co-authored-by: Craig MacKenzie <[email protected]>