Add grafana graph for large artifacts upload latency

Summary

As part of the corrective actions for incident 5521, grouptesting is in charge of setting up a way for us to view performance of uploading large artifacts (>= 1GB). We will do that by setting up a Grafana graph on our Verify:Testing dashboard.

Benefits

This will help us show that large artifact uploads need improvement. It will also allow us to track the performance improvements when we implement #285597 (closed).