[Core] Application metrics exposed by `ray.util.metrics` should optionally be namespaced under `ray_custom_.*`

[shomilj]

Description

Right now, application-level metrics from a workload + system-level metrics are merged together, which makes it a bit difficult to split these two streams of information for our two different user personas ("infra engineer" and "application engineer"). We would like to collect only infra-level metrics in System A, and application-level metrics in System B, but the only way we can do this is by explicitly whitelisting the system metrics right now. Namespacing (or optional namespacing) is better here.

Use case

See the description :)

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