Envoy uses https://github.com/circonus-labs/libcircllhist, the original design discussion was in envoyproxy/envoy#1965.

In the interest of minimizing total dependencies, we should answer "is there a strong reason to be using HDR histogram vs. this log-linear library?". This library may have already been considered, if we can articulate our rationale in this issue we can have some design history.