[MR Analytics] Improve sort performance by adding NULLS LAST to database index

Problem to solve

The Throughput chart on the MR Analytics page loads slowly. @ahegyi pointed out in #243709 (comment 439684522): "The sort is not supported (NULLS LAST) is missing from the index definition) by the original index on metrics: https://explain.depesz.com/s/S3fK"

Proposal

We should add NULLS LAST to the index definition on merge_request_metrics.