Alphabetize default metrics

Metrics are being reported in the order they were seen. This is a peculiarity of Object.keys() that persisted when we switched to Maps for metric storage. In conjunction with https://github.com/prometheus/client_js/pull/689, which will allow people to disable particular default metrics, and #539, which has a side effect of reducing the degree of label reordering between cluster_metrics calls, I think it would be useful from an ergonomics standpoint if someone reordered the defaultMetrics to register the metrics in order of their metric name. Since each of the metrics uses a common prefix for all of its own stats, this should mostly be a matter of rearranging the entries in the object in lib/defaultMetrics.js to order the prefixes alphabetically, and then inspecting the output from one of the examples to verify. Some defaults like heapSizeAndUsed may need a little tweak to make sure the first metric registered is the one with the collect() on it (currently that's total, which is alphabetically last).