Replace the existing systray_start counter metrics with a
systray_running gauge metrics.
This also adds an IncrementGauge method to local client to parallel
IncrementCounter. The LocalAPI handler supports both, we've just never
added a client method for gauges.
Updates #1708
Change-Id: Ia101a4a3005adb9118051b3416f5a64a4a45987d
Signed-off-by: Will Norris <will@tailscale.com>