This makes labels order independent in a way similar to what the
official Prometheus::Client now does:
https://github.com/prometheus/client_ruby/blob/7d9d45f547ce56ccf06344ab089e67228ddb7431/lib/prometheus/client/data_stores/direct_file_store.rb#L169

It shouldn't matter what order the labels were in for every change to
the metric's value or how the metrics were initialized. But since the
values were written out in 2 arrays before it did.

This works around that by sorting the label names and values
alphabetically by label name before writing out the key of the metric
into the file.