Summary: Add option to setup fibers' instrumentation.

Test Plan: Unit tests

Reviewed By: andrii@fb.com

Subscribers: folly-diffs@, yfeldblum, chalfant

FB internal diff: D1991638

Tasks: 6347990

Signature: t1:1991638:1429144487:3c3586b9943b4cc6bbef0297e48b8bffaf586f0d