There is a logging bug affecting mongos nodes in v3.6+: https://jira.mongodb.org/browse/SERVER-36801
In short when the -collect.collection option is used every call to collStats (or aggregation $collStats) prints 5 or so lines with an warning message "mongos collstats doesn't know about: xxxx". If you have say 100 collections then that's 500 log lines being added everytime the mongodb_exporter is run against the mongos node. This can lead to a disk-full incident soon enough.
I don't think we should run -collect.collection on a mongos node. The option used against the mongod nodes adds up to the same stats. So I think its fine to stop -collect.collection on mongos nodes in PMM. I suggest that as a fix to this issue.