Support query to determine largest metrics

I was trying to figure out what my biggest metrics were, per https://www.robustperception.io/which-are-my-biggest-metrics.

So I tried `topk(10, count by (__name__)({__name__=~".+"}))`, which failed with `Error executing query: invalid parameter "query": substitute queries failed: convert vector selector failed: series selector must specify an exact metric name`.

I'm not sure what part failed exactly, but I'm guessing it's the `__name__=~".+"` label selector. I'm guessing that might be hard to do in Monarch, so I don't know if it's reasonable to support that directly, or if there's a better way to support a way to determine which metrics are most expensive.