Hi,
System **** on port 1521 metrics indicate we only see a Socket call going outside the JVM which otherwise is not detected as a backend call by SQL Agent.
This could happen in a scenario for e.g when there is some application code that is making a socket call to Oracle backend using code that is not probed\monitored out of the box with SQL Agent extension or if database driver is uncertified.
This thread might also help:
Backend "System" Calls in Investigator
Regards,
Kulbir.