Add namespace to vector, make metrics public

[blind-oracle]

Add a namespace label to Vector metrics, export Metrics struct as public & allow creation of Vector client with pre-built metrics.

This is needed to run several instances of Vector client inside one process.

[Copilot]

Pull request overview

Adds per-instance metric separation for the Vector client by introducing a namespace label on all Vector metrics, and exposes a public Metrics type so multiple Vector clients can share a single registered metric set within one process.

Changes:

• Converted Vector metrics from single metric types to *Vec metrics, adding a namespace label (and ok for flushes).
• Updated Vector::new to accept a namespace and added Vector::new_with_metrics to allow injecting pre-built/shared metrics.
• Updated call sites in the module tests to pass a namespace and read labeled metrics.

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