Add tags/change namespace in scripting

[soulgalore]

A really powerful feature would be to set the namespace of a test in a script dynamically and add tags per test as proposed in #2512. I think they should be implemented the same time because it could probably be a generic thing.

The tags and namespace needs to be passed around in the messages in the queue (it could be some extra fields), that way Graphite/InfluxDb can override the default settings easily.

One way to implement it would be to add more meta data about a test https://www.sitespeed.io/documentation/sitespeed.io/scripting/#meta-data

We could add a "tag" feature or just key/value, then sitedpeed.io could pickup those extra meta data and pass them on in the messages. I'm not 100% sure yet how it should work, but I think that would be a way forward.

It would make scripting super generic and also make it possible to add annotations from scripting, so you could send a specific annotation depending on what happens in the script.

But lets add one thing at a time :)