AFAIK, you cannot create an alarm to monitor disk latency on a datastore. Also, even though the alarm for VM disk latency exists in vCenter, it does not work. This is a bug, and a PR is generated with VMware Support. Looks to me the next major release will offer disk latency monitoring at the VM level.
For now, the best tool for what you are looking to do would probably be 3rd party, something like akorri
Veeam monitor lets you set a custom alarm for datastore latency/io threshholds exceeded. Only available in the full version but worth every penny.
For now - this needs either a 3rd party tool - or some scripted monitor setup(if you have no budget) , to alert you
(e.g. you could use the get-esxtop PowerCli CMDLETs, and have them track performance every x amount of seconds . . then mail you etc in the event of an issue) - but this is still clumsy