Add support for custom user tags

[svenstaro]

You can set tags for monitors and monitor groups on site24x7. Support parsing these and add them as labels to the metrics.

[svenstaro]

Some progress in 75167b0

I can't use the MetricVecs for this any more as they need their labels predeclared and they can't change. Instead, I'll have to change it so all metrics get recreated from the ground up every time the metrics are collected and then I can choose any labels I want on every single scrape.