Adding hostname label from instance in Prometheus doesn't work

Question

I want to add a "hostname" label to my metrics in Prometheus, because I don't need domain or port information in my dashboards.

I tried adding this to the respective exporter settings in my Prometheus config:

relabel_configs:
- source_labels:
  - instance
  regex: "([a-z][a-z0-9-]*)(\\.[a-z][a-z0-9-]*)*(:[0-9]+)?"
  target_label: hostname

but it doesn't seem to have an effect. Even if I remove the regular expression, I don't get a label "hostname" in the scraped metrics when I would have expected a copy of the label "instance."

Answer 1

The label instance seems to be generated at a later point in time. For generating an additional hostname label use the special label __address__ instead:

relabel_configs:
- source_labels:
  - __address__
  regex: "([a-z][a-z0-9-]*)(\\.[a-z][a-z0-9-]*)*(:[0-9]+)?"
  target_label: hostname