stats: native prometheus export support #1947
Description
Things that need sorting out:
- Push vs. pull
- If we do native pull, Envoy will need a built-in histogram implementation. We need this for outlier detection also. The implementation I was planning on using is an HDR histogram implemented here: https://github.com/circonus-labs/libcircllhist (stats: add built-in log linear histogram support #1965)