Add support for prom-client v14

[leedm777]

The prom-client module released v14 on Sept 18. Please update swagger-stats to allow for the upgrade. The breaking changes in the prom-client release are:

• changed: linearBuckets does not propagate rounding errors anymore.

  Fewer bucket bounds will be affected by rounding errors. Histogram bucket labels may change. 6f1f3b2

• changed: The push gateway methods pushAdd(), push() and delete() now return Promises instead of accepting a callback:

  // Old:
  gateway.pushAdd({ jobName: 'test' }, (err, resp, body) => {});
  // New:
  gateway
    .pushAdd({ jobName: 'test' })
    .then(({ resp, body }) => {})
    .catch(err => {});
  // or
  const { resp, body } = await gateway.pushAdd({ jobName: 'test' });
  

  f177b1f

• changed: The default nodejs_eventloop_lag_* metrics are now reset every time they are observed. This prevents these metrics from "stabilizing" over a long period of time and becoming insensitive to small changes. For more info, see #370. 0f444cd