Loki: Add annotation support

[torkelo]

No description provided.

[ryantxu]

I am hoping we can replace the special annotation query with any query that returns:

{
time
text
endTime?
tags?
userInfo?...
}

Details still need worked out :)

[lorenwest]

Upvoting as we would prefer storing immutable system generated events/annotations in Loki than the mutable Grafana Annotations API

[aocenas]

Taking a stab at this. @davkal does it make sense to use label values as tags automatically for the annotations?

[aocenas]

@ryantxu

I am hoping we can replace the special annotation query ...

Would make sense imho, but I think right now with all the other changes going on with datasource API I will do this the old way and we can change it later on.

[davkal]

Yeah, i'm happy with label values being the tags in the annotation.

[ryantxu]

If we are fine replacing the work soon (hopefully 6.5) than I am +1 for trying

[lorenwest]

Thank you all for your work on this!