Decorate mpsc-notification-to-protocol with the protocol name (#3873)

Currently, all protocols use the same metric name for
`mpsc-notification-to-protocol` this is bad because we can't actually
tell which protocol might cause problems.

This patch proposes we derive the name of the metric from the protocol
name, so that we have separate metrics for each protocol and properly
detect which one is having problem processing its messages.

---------

Signed-off-by: Alexandru Gheorghe <[email protected]>