Error: com.mysql.cj.jdbc.Driver not loaded. Are you sure you've included the correct jdbc driver in :jdbc_driver_library?
for logstash 6.2.x and above, prefer adding the drivers under:
/usr/share/logstash/logstash-core/lib/jars/
and rather keep the jdbc_driver_library empty.
This has been quoted in many of the elastic.co forums too.