Inspecting the registry to see if there are any metrics to push

[bjornicus]

When pushing metrics on an interval, after doin a registry.reset(), if there have not been any new metrics recorded it would be nice to be able to skip pushing if all metrics are 0. As far as I can tell there isn't an easy way to check this, is there?

This is somewhat related to #40 in looking for convenient ways to make pushing metrics as efficient as possible by not pushing things we don't need to.