Deprecation warning for time unit :microseconds

[juulSme]

Hi there! Thanks for the great work on this lib, using it for the first time and I like it :) I found a small issue (I also fixed it, will send PR right after). It's a Phoenix 1.4 project with Elixir 1.8.0 so the paint is still fresh. The problem is a deprecation warning for time unit :microseconds, originating in the instrumenter code.

warning: deprecated time unit: :microseconds. A time unit should be :second, :millisecond, :microsecond, :nanosecond, or a positive integer
  (my_app) lib/my_appmonitoring.ex:29: MyApp.EctoInstrumenter.log/1
  (telemetry) /home/me/Projects/my_app/deps/telemetry/src/telemetry.erl:113:anonymous fn/4 in :telemetry.execute/3