Steps to reproduce:

1. System worsk initially as expected and monitors MongoDB
2. MongoDB hangs and stop responding
3. Zabbix Agent2 zabbix_get calls to MongoDB plugin related Item keys hangs too
4. Other zabbix_get calls like agent.ping works well

Example (IP address and password obfuscated)

1. time zabbix_get -s 0.0.0.0 -k mongodb.ping["tcp://localhost:00000","zab","xyz"]

zabbix_get [4087707]: Timeout while executing operation

real    1m0.003s

user    0m0.000s

sys     0m0.002s

Result:
Zabbix_get call timeouts after1 minute - network timeout.

The consequence is, that Zabbix marks whole interface on the monitored Host as unsupported because of the network timeout and after while Zabbix marks the Interface working again because other Items  like agent.ping works. 

This flapping causes huge load by "unsupported pollers".

Expected:
Expected result is that Zabbix agent returns Item value or returns ZBX_NOTSUPPORTED after agent timeout (3 secs) is reached.