Go (v1.13+) get github.com/coreos/etcd@v3.3.10 will fail if using proxy gocenter.io; Works with default proxy (proxy.golang.org)

This can be viewed as a bug or improvement.  

Using the gotip (go client version 1.13 and above):

• setting export GOPROXY=https://gocenter.io
• gotip get github.com/coreos/etcd@v3.3.10 returns
  • go: finding github.com/coreos/etcd v3.3.10
    go get github.com/coreos/etcd@v3.3.10: github.com/coreos/etcd@v3.3.10: reading https://gocenter.io/github.com/coreos/etcd/@v/v3.3.10.info: 404 Not Found
• curl https://gocenter.jfrog.io/github.com/coreos/etcd/@v/v3.3.10.info – returns not found 

Using go client default proxy - proxy.golang.org

• unset GOPROXY 
• get github.com/coreos/etcd@v3.3.10 will get pass the 404 error.  
• curl proxy.golang.org/github.com/coreos/etcd/@v/v3.3.10.info {"Version":"v3.3.10+incompatible","Time":"2018-10-10T17:17:54Z"}

Improvement

Request from the user perspective is to have go client (v1.13+) behave the same when using gocenter.io as using proxy.golang.org (default go proxy).  The request is to have artifactory (gocenter.io) returns the similar information when <module>.info is requested (returns the +incompatible).  

Caddyshack does this already when processing modules like github.com/coreos/etcd@v3.3.10 - it sees there are no /v3 on the module name, it will insert +incompatible next to the version v3.3.10