If you create a distribution but don't write to it, the mean of the distribution is NaN - which Aeson encodes as `null`. Thus we have to make sure that when working with the mean, we check for `null`.
The new package, ekg-core, contains the metrics store, modules for defining metrics, and functions for sampling the metrics. The counter naming is also changed to include a namespace part. For example "myapp.requests". Finally the JSON format has been changed to be more self-descriptive.