v2.2.4 — Live draws its numbers

Added

• The Live tab draws its numbers as well as printing them. Each gauge grows a sparkline of the
  last three minutes, kept in the tab — nothing stores gauge history, because the gauges are
  sampled when read, and the panel says so rather than implying it holds yesterday. A gauge that
  could not be read gets its dash and no plot: an empty plot area reads as flat at zero.
  (#274)
• Context used and its limit are one meter instead of two tiles. They were never two facts,
  and reading 428K against 1,048,576 is arithmetic the panel should do. The fill carries severity
  past 75% and 90%, with the percentage always spelled out — a status colour never carries meaning
  alone. An absent or zero limit reads as unknown rather than as 0%.
  (#274)
• Two charts under the gauges: calls per minute, derived from the difference between
  successive reads of the cumulative counters the way rate() does, and a latency histogram
  over the nine buckets. An empty latency bucket keeps its row, because a band with no calls is
  information. GET /api/webui/metrics/counters serves both.
  (#274)

Fixed

• The counters charts cannot take the Live tab down. They render inside it, so trusting the
  route's shape meant an older gateway — or any unexpected answer — unmounted every gauge above
  them. The same failure the scale row had one release earlier.
  (#274)