refactor(dp-server): remove metadata tracker

[lahabana]

Motivation

We were tracking the metadata in its own dataplane callbacks.
In practice this metadata was only used inside the sync_tracker which is itself a dataplane callback.

Implementation information

We now keep the metadata directly inside the sync_tracker and access it on each watchdog step.
This makes the code more streamlined and removes a hard to understand abstraction that added little value.

Supporting documentation

Part of #12881

[github-actions]

Reviewer Checklist

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• Is the PR title satisfactory? Is this part of a larger feature and should be grouped using > Changelog?
• PR description is clear and complete. It Links to relevant issue as well as docs and UI issues
• This will not break child repos: it doesn't hardcode values (.e.g "kumahq" as an image registry)
• IPv6 is taken into account (.e.g: no string concatenation of host port)
• Tests (Unit test, E2E tests, manual test on universal and k8s)
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[jijiechen]

Looks good to me.
But can I get this in first to have less conflicts? #12886

[lahabana]

Yes but I think the way we fix #12886 doesn't touch this anymore