[pkg-go] prometheus is marked for autoremoval from testing

[Debian testing autoremoval watch]

prometheus 2.6.0+ds-1 is marked for autoremoval from testing on 2019-02-20

It (build-)depends on packages with these RC bugs:
918618: golang-github-grpc-ecosystem-grpc-gateway: golang-github-grpc-ecosystem-grpc-gateway FTBFS: github.com/grpc-ecosystem/grpc-gateway/runtime [build failed]
920009: golang-google-cloud: golang-google-cloud FTBFS with golang-google-genproto-dev 0.0~git20190111.db91494-1