Kentoku Shiba (author of Spider storage engine) suggests metadata_lock_info plugin:

This plugin makes it possible knowing "who has metadata locks". In development stage, sometimes DBAs meet metadata locks when using alter table statement. This metadata locks are sometimes caused by GUI tools. In service stage, sometimes DBAs meet metadata locks when using alter table statement. This metadata locks are sometimes caused by long batch processing. In many cases, the DBAs need to judge immediately. So I made it for all DBAs.

Plugin is available at:
lp:~kentokushiba/maria/10.0.3-metadata_lock_info