According to the specification and documentation changes in ZBXNEXT-6144 new internal metric keys should return "0" or show an error when memory buffer is disabled (ProxyBufferMode=disk):

1. zabbix[proxy_buffer,buffer,<mode>] - should return the "Proxy memory buffer is disabled" error.
2. zabbix[proxy_buffer,state,changes] - should return "0".
3. zabbix[proxy_buffer,state,current] - should return "0" (disk mode).

Also I assume that in such case these metrics probably should be absent in the zabbix[stats,<ip>,<port>] key, like for other optional processes (connectors in Zabbix server metrics, discovery manager/worker/queue metrics...) when memory buffer is disabled.

Steps to reproduce:

1. Build Zabbix components from the dev branch (e6ff77f60bb).
2. Disable the memory buffer by deleting current parameter line (ProxyBufferMode=hybrid) or by setting its value to "disk".
3. Create and test new items on the Zabbix proxy host.

Result:
1. New items return some hardcoded data:

2. zabbix[proxy_buffer,buffer,total] returns 16 MB, even if you comment out "ProxyMemoryBufferSize=16M" or set the value to "0".

2. Internal statistics key also contains data about proxy memory buffer:

Expected:

New item keys should behave according to the specification and documentation.