Consistently treat a label with an empty value as non-existant

[beorn7]

Currently, the signature AKA fingerprint of a metric changes if a label with an empty value is added.

[beorn7]

Un-assigning from myself as I intend to focus on other Prometheus projects. Also, this is something that will change with Prometheus 2.x, where I have little involvement. I leave it to the experts to figure out how to deal with this issue.

[brian-brazil]

I've looked through the 2.x code, and we're handling this correctly on ingestion in mutateSampleLabels. Recording rules also look good.

However, alerting rules are not handling this correctly.