Every node stores its own gauge readings, and the dashboard took the
latest from whichever node wrote last, or added up every reading in a
chart slice. Per-node gauges (memory, open connections) showed one node
or a pile of ticks, and the cluster-wide counts could show another
node's drifted copy: a 20-digit queue (an underflow stored unsigned) or
0 accounts.
gaugeReading takes each node's latest reading, adds per-node gauges up,
and for queue/user/domain counts takes the largest believable one, which
also skips the junk already in history (inbuxa-server stops storing it).
Cards, chart slices and the memory tile use it; the tile now reads all
nodes from history rather than one node's live value.