[MR Analytics] Do not join on metrics table when no filters are given

Problem to solve

The Throughput chart on the MR Analytics page loads slowly. @ahegyi pointed out in #243709 (comment 439684522) that we could speed up the default page load by only querying merge_request_metrics and skipping querying merge_requests.

Proposal

Adjust the Throughput chart query so it queries only merge_request_metrics if there are no extra filter params present.