Add FortiGate DHCP Scope usage percentage sensors

[rudybroersma]

Here's a pull request to add DHCP usage percentage sensors. It gives a (per Virtual Domain) list of DHCP servers/scopes with their lease percentage. eg, a pool of 100 IPs with 80 active DHCP leases gives a usage percentage of 80%. This can be used for DHCP server utilization monitoring.

Sample output:

The name 'root' in this example is the FortiGate Virtual Domain name and is retrieved from the device.

dev:check succeeds. Updated FortiGate 40F test data.

ADDITIONALLY I made a small correction to a PR that was accepted earlier. I corrected the description in the file and removed an unused line of code here:

f6a42d4

NOTE: I believe this cannot be added using YAML, because the fgDhcpTable doesn't end with a single index.

# snmpwalk -v3 -u snmp 192.168.178.164 -M /opt/librenms/mibs:/opt/librenms/mibs/fortinet -m all fgDhcpTables
FORTINET-FORTIGATE-MIB::fgDhcpLeaseUsage.2.1 = INTEGER: 18
FORTINET-FORTIGATE-MIB::fgDhcpLeaseUsage.3.1 = INTEGER: 49

The values 2 and 3 belong to the vDOM, and 1 is the Server ID.
So in this example it's: vDOM 2, server ID 1 has a usage of 18% and vDOM 3, server ID 1 has a usage of 49%

DO NOT DELETE THE UNDERLYING TEXT

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[electrocret]

LGTM.
Thanks!

[librenms-bot]

This pull request has been mentioned on LibreNMS Community. There might be relevant details there:

https://community.librenms.org/t/24-2-0-changelog/23721/1