metrics: satisfy http.Hijacker
metrics: satisfy http.Hijacker

This change is necessary (but not sufficient) to fix chain/chainprv#1569.

Closes chain/chainprv#1574.
Reviewers: da39a3ee5e6b4b0d3255bfef95601890afd80709@bobg




chain/chain,chain/chain,chain/chain,chain/chain,chain/chain,chain/chain