Emit metric when using default SignalFX key as fallback (#782)
Emit metric when using default SignalFX key as fallback (#782)

* remove line that logs too frequently

* emit a globalized metric flush.fallback_client_points_flushed instead

* also tag "vary_by"

Co-authored-by: Randall Ma <b303809df130880a9c942350c555edddd5bdabb3@randallma.com>
stripe/veneur,stripe/veneur