BUG: Flaky MetricsProvider tests in CI

[ryan-roemer]

I'm anecdotally seeing a lot of failures like:

  68 passing (24s)
  2 failing
  1) MetricsProvider _onMetrics retains metrics received, while aggregating them into time buckets:
     Error: Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.
  
  2) MetricsProvider _onMetrics aggregates data for non-uniform data sets too:
     Error: Timeout of 10000ms exceeded. For async tests and hooks, ensure "done()" is called; if returning a Promise, ensure it resolves.

in Travis. Current incarnation: https://travis-ci.org/FormidableLabs/nodejs-dashboard/jobs/311447216 (I'm restarting this build so it may pass next time...)

[jjasonclark]

This failed again on build #78.2

[jjasonclark]

Fixed with PR change