refactor(metrics): avoid temporary buffer when serializing metrics

[Manciukic]

Changes

Metrics are currently serialized in a temporary buffer (String) before being written to the line buffered writer, which then writes finally to the file.
There is no need for this temporary buffer, as the metrics can be serialized directly into the writer.
We also need to manually write a newline as serde doesn't do that and we need it to separate the metric lines and flush the line writer.

Reason

I found this as I saw Firecracker was allocating and freeing memory every minute, causing pages to be mmap/munmap-ed periodically (and unnecessarily). This change removes these extra allocations, although the benefits are marginal at best as this action is only performed once a minute. Nonetheless, it's just 5 lines, so why not.

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

• I have read and understand CONTRIBUTING.md.
• I have run tools/devtool checkstyle to verify that the PR passes the
  automated style checks.
• I have described what is done in these changes, why they are needed, and
  how they are solving the problem in a clear and encompassing way.
• I have updated any relevant documentation (both in code and in the docs)
  in the PR.
• I have mentioned all user-facing changes in CHANGELOG.md.
• If a specific issue led to this PR, this PR closes the issue.
• When making API changes, I have followed the
  Runbook for Firecracker API changes.
• I have tested all new and changed functionalities in unit tests and/or
  integration tests.
• I have linked an issue to every new TODO.

---

• This functionality cannot be added in rust-vmm.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.98%. Comparing base (a02fc1e) to head (ac60da5).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5303      +/-   ##
==========================================
+ Coverage   82.91%   82.98%   +0.06%     
==========================================
  Files         250      250              
  Lines       26880    26873       -7     
==========================================
+ Hits        22288    22300      +12     
+ Misses       4592     4573      -19     

Flag	Coverage Δ
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5.10-m5n.metal	83.42% <100.00%> (+0.01%)	⬆️
5.10-m6a.metal	82.64% <100.00%> (+0.02%)	⬆️
5.10-m6g.metal	79.25% <100.00%> (+0.01%)	⬆️
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6.1-m5n.metal	83.46% <100.00%> (+0.01%)	⬆️
6.1-m6a.metal	82.69% <100.00%> (+0.01%)	⬆️
6.1-m6g.metal	79.25% <100.00%> (+0.02%)	⬆️
6.1-m6i.metal	83.46% <100.00%> (+0.02%)	⬆️
6.1-m7a.metal-48xl	82.67% <100.00%> (?)
6.1-m7g.metal	79.24% <100.00%> (+0.01%)	⬆️
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6.1-m8g.metal-24xl	79.23% <100.00%> (?)
6.1-m8g.metal-48xl	79.24% <100.00%> (?)

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