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

https://docs.victoriametrics.com/vmagent.html#relabeling-enhancements