Add sharding key to resource_label_events

Mario Celirequested to merge
514594-add-resource-label-events-sharding-key into master

What does this MR do and why?

Adds namespace_id column to resource_label_events table as it's sharding key. In this MR we make sure every new records sets the correct sharding key. The table will be backfilled in a follow-up MR

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Related to #514594 (closed)

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