vlib: calculate per-worker loops/second metric

Use exponential smoothing. Each sample has a half-life of 1
second. reported_rate(t) = reported_rate(t-1) * K + rate(t)*(1-K)

Sample every 20ms, i.e. 50 samples per second
  K = exp (-1.0/20.0);
  K = 0.95;

Type: feature

Signed-off-by: Dave Barach <dave@barachs.net>
Change-Id: I9aea5dd5fecfaefffb78245316adb4bf62eb2bd4