tperf: histogram of bytes per stream
tperf: histogram of bytes per stream

Summary: per stream stats.

Reviewed By: mjoras

Differential Revision: D18345434

fbshipit-source-id: e847ca7af18f03759da96a0fa98f5be037737d9a

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst