Yikes, I just noticed that since we persist all graphs with their expressions, this also saves the entire metrics list on the server (per expression).

So we need to attach the list somewhere outside of what gets saved. Maybe if the `expression` directive got an isolate scope (with the expression passed in), you could just stick it onto each expression's scope? Or maybe there's an easier solution, hmmm...

I noticed this due to another bug: when going to a dashboard which already has expressions which were saved *without* their metrics lists, the metrics aren't loaded at all for these expressions.