[a=bicycleday] Preserve gauges after each flush.

If the gauge is not updated at the next flush, it will send the
previous value. This is the definition of a gauge as per Etsy StatsD.
Also updated test_flush_gauge_metric in test_processor.py.
[r=sidnei] [a=bicycleday] Preserve gauges after each flush.

If the gauge is not updated at the next flush, it will send the
previous value. This is the definition of a gauge as per Etsy StatsD.
Also updated test_flush_gauge_metric in test_processor.py.