Receive: Add exported_ prefix to labels conflicting with exposed --label

[ahurtaud]

Is your proposal related to a problem?

We have few thanos receive instances with some announced labels using the --label flag.
example:

      --label=prometheus="shared"
      --label=receiver="shared"

When clients remotewrite points to the thanos receive instance containing conflicting labels, lets say prometheus="k8s".

Those labels are overriden in the query path. -> the metrics loss its prometheus="k8s" labels and the queriers replies to prometheus="shared".

Describe the solution you'd like

I would like to have the same mechanism of prometheus scraping conflicting labels, adding the exported_ prefix. (and maybe bring in all the honor_labels mechanism with it)

so to continue with my example, the metrics would be available with:

{prometheus="shared', exported_prometheus="k8s"}

Describe alternatives you've considered

Short term we will ask the clients to change their labels to have non conflicting with the ones announced by receive, but I feel we will face other occurences regularly on this.