Compute average costs more correctly in Multimeter

Summary: Ref T6930. We were dividing total cost by number of samples, but should more accurately divide it by the sum of the sample rates. This produces a more meaningful number.

Test Plan: Saw a more meaningful number in the UI.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6930

Differential Revision: https://secure.phabricator.com/D12655