Harvest Poller panics from ZapiPerf:Volume collector

[hashi825]

Describe the bug
Harvest Poller panics from ZapiPerf:Volume collector and doesn't recover

Environment
Provide accurate information about the environment to help us reproduce the issue.

• Harvest version: harvest version 21.05.2-1 (commit ce091de) (build date 2021-06-14T20:31:09+0530) linux/amd64
• Command line arguments used: [e.g. bin/harvest start --config=foo.yml --collectors Zapi]
• OS: RHEL 7.9
• Install method: yum
• ONTAP Version: 9.7
• Other:

To Reproduce
This seems to be due to when an instance goes missing from cache, which by default instance cache is 600s, the times I've experienced this issue is when a volume has been deleted. This particular occurrence a volume got deleted from the relevant cluster roughly 10mins before the panic occured.

Expected behavior
Poller should not panic when a volume gets deleted

Actual behavior
ZapiPerf:Volume collector causes a panic when volume gets deleted.

Possible solution, workaround, fix
N/A

[rahulguptajss]

Thanks @hashi825 . THis is similiar to #174 . I am working on the fix for the same.