feat(metrics): store dimensions alongside metrics in store_data

[Manciukic]

Changes

This patch changes the format of the metrics.json to add two keys: metrics, contanining the old dict of metrics, and dimensions, contiaining the dict of dimensions.
We're not storing all other properties as they are the same across all tests on the same executor and they can be fetched from test_result.json already, under "user_properties".

The metrics.json is not used anywhere in the codebase (except some downstream scripts), so this format change won't break anything.

Example:

{
  "metrics": {
    "throughput_guest_to_host": {
      "unit": "Bits/Second",
      "values": [
        1521035552.397601,
        1509913260.8668907,
        1516511907.9876878,
        1519015711.4238577,
        1521465457.3455591,
        1518696862.9045,
        1515518147.2383862,
        1522867838.40718,
        1527683628.617747,
        1541685794.8929434,
        1545028848.734238,
        1541926412.701195,
        1533085549.7164657,
        1530072366.862809,
        1526355028.8311164,
        1521260173.491227,
        1511490591.6653512,
        1546977580.4738607,
        1515960423.8909352
      ]
    },
    "throughput_host_to_guest": {
      "unit": "Bits/Second",
      "values": [
        5034717094.48179,
        5014916646.993628,
        5049309117.50637,
        5051670826.80042,
        5038316942.29528,
        5038833664,
        5040061261.798115,
        5011715459.06281,
        5031551121.01712,
        5092404817.727023,
        5082481599.609217,
        5080387407.336452,
        5072570762.918288,
        5071266247.682884,
        5081399296,
        5070318876.56711,
        5032219627.042276,
        5070049733.85223,
        5060680852.0309
      ]
    }
  },
  "dimensions": {
    "performance_test": "test_vsock_throughput",
    "payload_length": "64K",
    "mode": "bd",
    "instance": "m5n.metal",
    "cpu_model": "Intel(R) Xeon(R) Platinum 8259CL CPU @ 2.50GHz",
    "host_kernel": "linux-5.10",
    "guest_kernel": "linux-5.10",
    "rootfs": "ubuntu-24.04.squashfs",
    "vcpus": "2",
    "guest_memory": "1024.0MB"
  }
}

Reason

In d6e5521b we added a metrics.json to dump all emitted metrics, but we don't add the dimensions to easily parse and know to know to what they refer.
We may want to switch our ab_test framework to switch using this file at some point, but that's not really needed.

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 checkbuild --all to verify that the PR passes
  build checks on all supported architectures.
• 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 83.03%. Comparing base (1dac3b6) to head (e0e262b).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5329      +/-   ##
==========================================
+ Coverage   82.98%   83.03%   +0.05%     
==========================================
  Files         250      250              
  Lines       26837    26837              
==========================================
+ Hits        22271    22285      +14     
+ Misses       4566     4552      -14     

Flag	Coverage Δ
5.10-c5n.metal	83.48% <ø> (ø)
5.10-m5n.metal	83.48% <ø> (ø)
5.10-m6a.metal	82.70% <ø> (-0.01%)	⬇️
5.10-m6g.metal	79.15% <ø> (ø)
5.10-m6i.metal	83.48% <ø> (ø)
5.10-m7a.metal-48xl	82.69% <ø> (?)
5.10-m7g.metal	79.15% <ø> (ø)
5.10-m7i.metal-24xl	83.44% <ø> (?)
5.10-m7i.metal-48xl	83.45% <ø> (?)
5.10-m8g.metal-24xl	79.14% <ø> (?)
5.10-m8g.metal-48xl	79.14% <ø> (?)
6.1-c5n.metal	83.53% <ø> (ø)
6.1-m5n.metal	83.53% <ø> (+<0.01%)	⬆️
6.1-m6a.metal	82.75% <ø> (ø)
6.1-m6g.metal	79.15% <ø> (ø)
6.1-m6i.metal	83.53% <ø> (ø)
6.1-m7a.metal-48xl	82.74% <ø> (?)
6.1-m7g.metal	79.15% <ø> (ø)
6.1-m7i.metal-24xl	83.55% <ø> (?)
6.1-m7i.metal-48xl	83.54% <ø> (?)
6.1-m8g.metal-24xl	79.13% <ø> (?)
6.1-m8g.metal-48xl	79.14% <ø> (?)

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