Error reported when plugin catalog is empty

[marcin-krolik]

Reproduction steps:

1. Start snapd eg. snapd -t 0 -l 1&
2. Check metric catalog (no collectors loaded) snapctl metric list

Results:
Snapctl reports error

$ snapctl metric list
Error getting metrics: metric catalog is empty, no plugin loaded

Snapd reports error

ERRO[2016-09-22T12:56:08+02:00] error fetching metrics                        _block=fetch _file=metrics.go, _module=control error=metric catalog is empty, no plugin loaded

This should not be reported as error, maybe just info, or warning?

During large tests pytest module checks snap output for any error, and fails the test if any error was found. While this could be easily changed to omit/discard errors returned by snapd, I think it's not beneficial to large tests in general

[IRCody]

If the reporting is changed to not be an error, what do you think the snapctl behavior should be? I think right now it would end up printing out the column header info then nothing which is not ideal.

[lynxbat]

Yeah, this is not an error. Asking for a list of metrics should be able to return an empty list.
I wouldn't mention plugins at all in the response as it is not really relevant.

[geauxvirtual]

This is definitely a change in behavior with the merge of PR #1220. Prior to this PR, an empty list would be returned to snapctl, and snapctl would just print out the column headers.

[lynxbat]

Functionally a REST API call to a list can return an empty list without error.

Snapctl can be a bigger discussion. I would prefer maybe a "helper" message after the empty list that can say "Try loading a plugin first to get some metrics"

[IRCody]

I submitted a PR for this and opened #1233 to track improvements to snapctl dealing with empty lists for metrics/plugins.