Prototype targets.merge function

[ptodev]

I hope this can enable users to decorate prometheus.exporter metrics with labels from discovery components.

The config could look like this:

discovery.kubernetes "default" {
  role = "service"
}

prometheus.exporter.unix "default" {}

// Probably we'd need some discovery.relabel to make sure both targets have a common key/val to serve as condition of joining

prometheus.scrape "demo" {
  targets    = targets.merge(prometheus.exporter.unix.default.targets, discovery.kubernetes.default.targets, ["__meta_kubernetes_service_cluster_ip"])
  forward_to = [prometheus.remote_write.mimir.receiver]
}

prometheus.remote_write "mimir" {
  endpoint {
    url = "https://prometheus-prod-05-gb-south-0.grafana.net/api/prom/push"

    basic_auth {
      username = ""
      password = ""
    }
  }
}

Which issue(s) this PR fixes

Fixes #1443

[mattdurham]

I wonder on the naming, the name makes me feel like its very SQL like and should behave like a SQL inner join.

[ptodev]

I wonder on the naming, the name makes me feel like its very SQL like and should behave like a SQL inner join.

@mattdurham Good point - I renamed it from "inner_join" to "merge". I also changed the namespace from "map" to "targets", because I realised that even "map" is not an appropriate namespace for this function as it operates on arrays of maps.

[ptodev]

Currently, the "third input" assumes both targets have a label with the same value. I wonder if it's worth changing the third input to be an array of arrays, where we can set different label names in the LHS and RHS targets. E.g.:

[["lhs_lbl_name"], ["rhs_lbl_name"], ["lhs_lbl_name_2"], ["rhs_lbl_name_2"]]

This could reduce the amount of relabels a user would have to do. But I don't know if it's really worth the extra complexity. The users probably have to relabel anyway.

[mattdurham]

What is the result for invalid input? Imagine the first is a valid map list but the second is nil/invalid type/etc? I imagine it would return the map list, whereas if it is the reverse it would return a blank list?

[mattdurham]

This feels off, whats the use case here?

[mattdurham]

Overall looks good! Though the complexity of this feels like it borders on a component, which means we could use the live debugging and mark as experimental. Since this has become specific to targets thoughts to add it to relabel.discovery?