Why external labels don't accept UTF-8 encoded string?

[Steve Wu]

I understand labels in prometheus metrics have to be as small as possible to save disk space, therefore we don't support UTF-8, but what about external labels used by alerting rules? Do we need them to be small too?

[Brian Candler]

That's simply not true. Firstly, "Label values may contain any Unicode characters" - https://prometheus.io/docs/concepts/data_model/

Secondly, whether a label value contains UTF-8 characters or only ASCII characters has no bearing as to whether it's "long" or not.  Thirdly, the length of label values has minimal impact on disk space, since they are stored only once.

Therefore, if you've seen some behaviour that you didn't expect relating to length or character set of labels, can you show exactly what you did, and what you observed?