Since 1.13 the behaviour of has been changed.

For metrics the _counter prefix has been removed, as is well documented and the snake case convention remains:

But for labels snake case has been removed in #4866:

But even though Prometheus client supports dots in metrics and labels since prometheus/client_java@1a955b1 , it still replaces dots with underscores via https://github.com/prometheus/client_java/blob/5e93f4b2af0321061865bad1f39f531f3d19bd0e/prometheus-metrics-model/src/main/java/io/prometheus/metrics/model/snapshots/Labels.java#L119-L130

For people using PrometheusNamingConvention in their own code the change to label handling is undocumented in the migration guide and actually the java doc for the method has an incorrect regex (without .):

Now, I am generally confused with the . and _ changes in Prometheus where they are allowed and yet... not allowed really? Maybe you know more about this.

I think it is a good idea to either mention the change regarding labels in PrometheusNamingConvention in the guide and fix the javadoc, or revert the snake case change to align with the way metrics are evaluated. To me the latter is preferable.

Let me know what you think, thanks!