Add noteable context to Note keep-around metrics
What does this MR do and why?
I've been trying to come up with an architecture for replacing keep-around references with scoped refs. We currently have prometheus metrics to tell us what models are creating these refs, but notes can be associated with a few different things. To help me finalise things here I'm adding this noteable type to the keep-around metrics.
Keep-arounds are disabled in tests and we don't seem to currently have any assertions. Since this is just for metrics labels I'm not too worried.
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