Add test coverage for passing sample_rate to LabeledStatsdClient
methods.
Drive-by: clean up and tighten some existing StatsdClient unit tests to
share a CommonBaseTestsMixin, as was expedient perhaps to the chagrin of
the co-author.
Related-Change: I115ffb1dc601652a979895d7944e011b951a91c1
Co-Authored-By: Alistair Coles <alistairncoles@gmail.com>
Change-Id: I68a24d9467e58994abbac82a5e741f5bb578976c
ed0ef35713