In all session where you want to use pg_injection you will have to load the extension using:
LOAD 'pg_injection.so';
Also, You may set error level (default is WARNING
)
SET pg_injection.level = ERROR;
If you want to generalize the use of the extension modify your postgresql.conf to set
session_preload_libraries = 'pg_injection'
or even
shared_preload_libraries = 'pg_injection'
and add
pg_injection.level = ERROR
at end of the file.