Replace use of default registerer in flagext package

[56quarters]

We're trying to move away from global metrics since this is supposed to be a shared library. @pstibrany recommended potentially setting the register and logger with some sort of init method that could be called from an application's main function.

[pracucci]

What @pstibrany suggested sounds a reasonable solution to me. I would keep the init function restricted to the flagext package tho. If we make it generic, it's more likely we'll end up using global logger/registered in other places, which is something we want to strongly discourage.

[aknuds1]

Agree we want to discourage use of global logger/metrics registerer.