allow dynamic strings to be used in metrics_format option

[elasticsearch-release]

Jenkins standing by to test this. If you aren't a maintainer, you can ignore this comment. Someone with commit access, please review this and clear it for Jenkins to run; then say 'jenkins, test it'.

[andrewvc]

Apologies @aaronhanson , I somehow missed this PR. This functionality was just put in by me in an accidental case of duplicate work :(. We'll try to be better about it next time!

[aaronhanson]

Cool, good to know.