[Data] Fix wrong metrics group for Object Store Memory metrics on Ray Data Dashboard

[scottjlee]

Why are these changes needed?

Currently, there is a bug where the Ray Data Metrics (Object Store Memory) charts do not render any data at all:

This is due to an incorrect metrics group used to initialize Prometheus metrics -- it should match "object_store_memory" used here.

After the fix in this PR, we see the charts populate (the workload used to generate the image is small, so there is no object spilling / significant objects piling up in the inqueue/outqueue):

Related issue number

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