Alert GeneratorURLs generated by the ruler do not indicate tenant

[leth]

The ruler generates the GeneratorURL from some static strings, which do not include the (internal) instance ID, and do not have a means of including the the user-facing instance ID.

The base URL defaults to "" , which gives us values like "generatorURL": "/graph?...".
It is plumbed through here:

• https://github.com/weaveworks/cortex/blob/773fcf1aeff0f99a5b51cf3148034b9e227435c2/pkg/ruler/ruler.go#L202
• https://github.com/weaveworks/cortex/blob/773fcf1aeff0f99a5b51cf3148034b9e227435c2/pkg/ruler/ruler.go#L283
• https://github.com/weaveworks/cortex/blob/773fcf1aeff0f99a5b51cf3148034b9e227435c2/pkg/ruler/ruler.go#L309

[bboreham]

Note that Cortex does not currently know the externally-visible instance ("user") ID we use in Weave Cloud, and it would be ugly for the ruler to make a service call to obtain it.

I went through some gyrations to avoid fixing this when integrating alert notifications. Maybe both IDs should just be copied into the Cortex config DB ?