Prevent AttributeError when Proxmox Metric Server is not configured

[alexeypilyugin]

What does this PR do?

Handle non-200 HTTP response from /cluster/metrics/export when Proxmox Metric Server is not configured to prevent AttributeError.

Motivation

AGENT-15529

Review checklist (to be filled by reviewers)

• Feature or bugfix MUST have appropriate tests (unit, integration, e2e)
• Add the qa/skip-qa label if the PR doesn't need to be tested during QA.
• If you need to backport this PR to another branch, you can add the backport/<branch-name> label to the PR and it will automatically open a backport PR once this one is merged

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.05%. Comparing base (579826f) to head (7b9d79b).

Additional details and impacted files

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