dispatch: don't allow nil registry for metrics

[simonpasquier]

No description provided.

[roidelapluie]

This is a patter we have in prometheus too. Why do you want to get rid of it?

[beorn7]

Indeed. It helps for testing and such.

I don't quite buy the argument of accidentally passing nil. Writing NewDispatcherMetrics(nil) is quite explicit and shouldn't happen accidentally (or you could say that even if nil is disallowed, you could still "accidentally" pass in a no-op registry).

[simonpasquier]

Oki doki. I'll close it then...