Conditionally drop labels

Question

I would like to drop a label but only from a specific series. From the doc I can see that the action needed is labeldrop.

My starting point is something like

    - action: replace
      source_labels: [__name__, url_rule]
      regex: 'test_.+;(.+)'
      target_label: path
    - action: labeldrop
      regex: 'url_rule'

How can I ensure that the second action will only drop the url_rule label from metrics starting by test_ (like in the first action)?

Answer 1

Prometheus doesn't provide the ability for conditional label removal via action: labeldrop. This action is applied unconditionally to every label of every metric. But you can use the following workaround for removal of url_rule label from metrics with names starting from test_:

- source_labels: [__name__, url_rule]
  regex: "test_.+;.+"
  replacement: ""
  target_label: url_rule

It just sets an empty value to url_rule label for metrics with names starting from test_ prefix. Then Prometheus drops labels with empty values after the relabeling.

P.S. The relabeling rule can be simplified with if condition when using VictoriaMetrics - Prometheus-like monitoring solution I work on:

- if: "{__name__=~'test_.+'}"
  action: labeldrop
  regex: url_rule

See these docs for details.