Using the excellent box292_check_vmware plugin and check_esxi_hardware.py script, you can get really great visibility of your ESXi servers.

Notes
For the box293 plugin, you need to have a VMA (details in the manual below).

For the ESXi Hardware you need to install pywbem (apt-get install python-pywbem) and on ESXi 6.5 you need to enable wbem (on the server type: esxcli system wbem set –enable true)

Firewall ports: 443 & 5989 are needed between the monitoring server, VMA and ESXi Hosts

ESXi Checks
Add a server on the VMA – port 443 needs to be open

/usr/lib/vmware-vcli/apps/general/credstore_admin.pl add --server IP.ADDRESS  --username root 

Check it works

/usr/lib/nagios/plugins/check_by_ssh -E 1 -l vi-admin -H VMA.IP -C "~/box293_check_vmware.pl --server ESXi.IP --check Host_OS_Name_Version"

Host Datastore Check

/usr/lib/nagios/plugins/check_by_ssh -E 1 -l vi-admin -H VMA.IP -C "~/box293_check_vmware.pl --server ESXi.IP --check Datastore_Usage --name "ESXI Datastore Name" --warning datastore_free:750 --critical datastore_free:700"

$ARG1$ server
$ARG2$ datastore name
$ARG3$ warning Gb
$ARG4$ critical Gb

Host Snapshot(s) Check

/usr/lib/nagios/plugins/check_by_ssh -E 1 -l vi-admin -H VMA.IP -C "~/box293_check_vmware.pl --server ESXi.IP --check Guest_Snapshot --host ESXi.IP --warning snapshot_age:5 --critical snapshot_age:15"

$ARG1$ Server
$ARG2$ Warning
$ARG3$ Critical