I would be nice to use slf4j instead of relying explicitly on log4j. (slf4j.org)
There are adapters from slf4j to many logger implementations, one of them is log4j.
The decision of which log engine does not belong in a library, it belongs to the user of that library.
This was apparently done already by several Apache projects. Searching for slf4j in Jira shows Zookeeper, HBase, HDFS, Parquet, Cloudstack, Nemo, EmpireDB, Oozie, Hadoop, and many others.
I can offer a patch, I have done such migration before.
Thank you very much,