Use double for values in Elasticsearch meter registry

[izeye]

This PR changes to use double for values in Elasticsearch meter registry.

Closes gh-1071
Closes gh-1420

[izeye]

There's a caveat. If the metrics_template template already exists, it needs to be deleted first before running. Another name could be chosen but the old one should be cleaned up anyway.

[shakuzen]

There's a caveat. If the metrics_template template already exists, it needs to be deleted first before running. Another name could be chosen but the old one should be cleaned up anyway.

I've tagged this with release notes so it can be mentioned in the release notes. Thank you for pointing it out.