Discoverer should log that it has started before doing other things

The first thing every Zabbix agent, proxy and server process does is reports that it has started in log file. It is very convenient for identifying process type by pid without necessity to look into configuration file parameters and count processes.

The only exception to this rule is server's discoverer process which does init_snmp() first. If something bad happens in init_snmp() for discoverer (see ZBX-10656, ZBX-11017) it's hard to tell from the first look which Zabbix process is to blame.