monitoring: handle metrics with many-many labels

[mberhault]

Some metrics will need a potentially very large set of labels, or even multiple labels. We need to find a good way of storing and accessing those. Emphasis should be on keeping metric updates cheap.

Along with this, the prometheus exporter should keep prometheusgo.Metric objects between iterations and simply call our internal metrics to update them.

[spencerkimball]

I saw a TODO in the code in prometheus_exporter.go with your name on it. But this doesn't actually seem to be a problem we have in practice. This feels like a solution looking for a problem. Closing for the time being as we don't need an issue to track this until it becomes an actual problem.