Pin github.com/prometheus/client_golang to v1.1.0
Pin github.com/prometheus/client_golang to v1.1.0

Version 1.2.0 of this library consumes github.com/cespare/xxhash/v2,
which we cannot pull from Glide.

Until we migrate to Go modules, we'll have to stick to older versions.

yarpc/yarpc-go,yarpc/yarpc-go