fix(test): avoid parsing broken metrics lines

[pb8o]

Changes

Ignore metric lines that cannot be parsed.

Reason

Improve the stability of tests.

License Acceptance

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license. For more information on following Developer Certificate of Origin and signing off your commits, please check CONTRIBUTING.md.

PR Checklist

• [x] If a specific issue led to this PR, this PR closes the issue.
• [x] The description of changes is clear and encompassing.
• [x] Any required documentation changes (code and docs) are included in this PR.
• [x] API changes follow the Runbook for Firecracker API changes.
• [x] User-facing changes are mentioned in CHANGELOG.md.
• [x] All added/changed functionality is tested.
• [x] New TODOs link to an issue.
• [x] Commits meet contribution quality standards.

---

• [x] This functionality cannot be added in rust-vmm.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 82.13%. Comparing base (c98ecc8) to head (af79a89).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4613   +/-   ##
=======================================
  Coverage   82.13%   82.13%           
=======================================
  Files         255      255           
  Lines       31267    31267           
=======================================
  Hits        25682    25682           
  Misses       5585     5585           

Flag	Coverage Δ
4.14-c5n.metal	79.63% <ø> (ø)
4.14-c7g.metal	?
4.14-m5n.metal	79.61% <ø> (-0.01%)	:arrow_down:
4.14-m6a.metal	78.84% <ø> (-0.01%)	:arrow_down:
4.14-m6g.metal	76.68% <ø> (ø)
4.14-m6i.metal	79.61% <ø> (ø)
4.14-m7g.metal	76.68% <ø> (?)
5.10-c5n.metal	82.15% <ø> (ø)
5.10-c7g.metal	?
5.10-m5n.metal	82.14% <ø> (+<0.01%)	:arrow_up:
5.10-m6a.metal	81.44% <ø> (ø)
5.10-m6g.metal	79.46% <ø> (ø)
5.10-m6i.metal	82.13% <ø> (+<0.01%)	:arrow_up:
5.10-m7g.metal	79.46% <ø> (?)
6.1-c5n.metal	82.15% <ø> (+<0.01%)	:arrow_up:
6.1-m5n.metal	82.13% <ø> (-0.01%)	:arrow_down:
6.1-m6a.metal	81.45% <ø> (ø)
6.1-m6g.metal	79.45% <ø> (ø)
6.1-m6i.metal	82.13% <ø> (ø)
6.1-m7g.metal	79.45% <ø> (?)

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