Tag replacement in dashboard can break some graphs

Right now with tag substitution you can add a tag to all graphs. If a particular metric doesn't have that tag it will cause it not to load.

One suggestion may be that tags passed in on the command line only _replace_ tags if they already exist in the metric and never add them to metrics where they previously didn't exist.

The use case we have is that we have a mysql dashboard where we want to sub host & port tags, but we also have the proc.stat.cpu.percent_used which is global to the server and doesn't have a port tag.