Support matchers that are only the added promxy labels
Support matchers that are only the added promxy labels

Promxy supports adding labels and filtering by them, if a query included
only matchers on these added labels the matchers would end up being
empty -- but downstream prometheus doesn't support empty matchers. This
simply replaces empty matchers with `{__name__=".+"}` (which effectively
matches all).

Fixes #251

jacksontj/promxy,jacksontj/promxy