Support sampling subprocess execution in Multimeter

Summary: Ref T6930. Add subprocess sampling/labeling.

Test Plan:
(This table includes some buggy test labels.)

{F391655}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6930

Differential Revision: https://secure.phabricator.com/D12675