Load labels count instead of labels for throughput table

What does this MR do?

Investigating for #243709 (closed), found out a possible improvement.

It seems that for the Throughput table on MR analytics for project page we need only count of labels for each MR and we don't have to load the labels themselves, which is the current state.

There is currently a bug #259678 which causes N+1 queries.

Fixing it will remove N+1 also here.

Screenshots

Does this MR meet the acceptance criteria?

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