Go mod: explicit golang.org/x/net replace

[jpmcb]

This PR is in reference to: #1952

From my fork:

➜ go mod tidy -v
get "golang.org/x/net": found meta tag get.metaImport{Prefix:"golang.org/x/net", VCS:"git", RepoRoot:"https://go.googlesource.com/net"} at //golang.org/x/net?go-get=1
go: go.etcd.io/etcd@v0.0.0-20190815204525-8f85f0dc2607 requires
	golang.org/x/net@v0.0.0-20190813000000-74dc4d7220e7: invalid pseudo-version: does not match version-control timestamp (2019-08-13T14:13:03Z)

After updating go.mod with my changes:

➜ go mod tidy -v
go: downloading google.golang.org/grpc v1.25.1
go: downloading github.com/prometheus/prometheus v1.8.2-0.20191126064551-80ba03c67da1
...

And is successful!

I've run go mod tidy && go mod vendor with the included changes. Also checked everything with make test. Let me know if this needs anything else!!

[CLAassistant]

All committers have signed the CLA.

[owen-d]

Thanks for this!