Can't get metrics: `ValueError: env prometheus_multiproc_dir is not set or not a directory` about mlserver HOT 1 CLOSED

I figured out the reason. I'll leave the information here for now, in case it'll be helpful for anybody else.
I had seldon-core package installed in the image as a leftover from previous experiments with seldon deployments. Its version was 1.18.0 (latest for v1 at the time). As one of its requirements, it references package prometheus-client<0.9.0 and >=0.7.1 so it prevents anything higher than 0.8.0 to be installed. But that version only checks env var prometheus_multiproc_dir (in lower case) as can be seen here. MLServer version 1.3.5 sets up this variable in upper case, as can be seen here. So prometheus-client should be of version >=0.10.0 to check both cases. Removing seldon-core package allows it to be installed in version 0.17.0.

from mlserver.