Display specific error messages when cloud-config fails

[rfratto]

PR Description

Changes the cloud-config command to attempt parsing the response body before returning an error that the response code was unexpected.

Which issue(s) this PR fixes

Fixes #360
Closes #629 (dupe of #360)

Notes to the Reviewer

PR Checklist

• CHANGELOG updated
• Documentation added
• Tests updated

[bigpresh]

Am I being blind - I don't see where you're actually outputting any extra info here?

The return "", fmt.Errorf("unexpected status code %d", resp.StatusCode) looks like the same as before; did you mean to add something from the parsed response into it? Or would the addition of dec.SetStrict(true) cause it to omit useful info (presumably only if the problem is that the response didn't contain valid YAML, but not if it did?

I'm not a Go coder, and I'm not very familiar with the Grafana Agent, so I may well be completely missing something obvious here.

I've tested it, but I'm getting the same result I did before - just could not retrieve agent cloud config: unexpected status code 500

I realised just after hitting submit on that comment that, whilst I'm pointing curl at the install script from the main branch, it is of course still grabbing the exact same deb package which won't have this change in - duh!

[rfratto]

I don't see where you're actually outputting any extra info here?

It's (unfortunately) hidden from the diff but the extra info gets printed out a few lines below. That logic was always there, but was being cut out by me prematurely reading the response code.

The new flow here is:

1. Try to parse the response as JSON.
2. If parsing fails, and the result wasn't 200, return the status code as a best-effort error message. Otherwise, return the failure about reading the response body. This shouldn't happen in practice.
3. If parsing succeeds, and the response JSON represents an error message, return that underlying error message, which is the part that was being hidden before. (If the response JSON doesn't represent an error message, use the success payload as the config file, returning successfully)

Once we do a release (sometime next week), the new error message will be could not retrieve agent cloud config: request was not successful: <issue from server>. Until then, if you're willing to hack around with things, you can manually run agentctl from the main branch :)

[bigpresh]

Ohhh, I see - yeah, that makes sense now, thanks for the explanation!

I'll have a crack at building from the main branch and seeing if it gets me any more info.