Convert all metric tags to lists

[jrconlin]

tags can only be lists.
In addition, tag composition could be made easier using a helper method like:

def maketags(base, **kwargs):
    # type: (Sequence[str], **Any) -> Sequence[str]
    tags = list(base)
    tags.extend('{}:{}'.format(key, val) for key, val in kwargs.iteritems())
    return tags

which would allow for things like
self.metrics('thing', tags=maketags(self._base_tags, foo=bar, gorp=1)

[jrconlin]

closed as part of #948