Remove Stackdriver exporter

[semistrict]

The Stackdriver exporter has been moved to a different repo:
https://github.com/census-ecosystem/opencensus-go-exporter-stackdriver

New import path: contrib.go.opencensus.io/exporter/stackdriver

Fixes: #789

[rakyll]

I have a question. Is it possible to keep this package with a doc.go and say it has moved to the new location for now?

Then it will break the builds and people will read the godoc and migrate to the new import path. Then, in a few releases, we can delete the package entirely.

[semistrict]

Yes, I will do that.

[odeke-em]

Could we gradually sunset this change say in a couple of weeks? I ask because OpenCensus has gotten a bunch of attention lately, and this change is going to break a bunch of apps that I and others have freshly pushed out to encourage people to check it out, but also as quick starts. A gradual sunset gives me and others time to go update our source code, say in 2 or 3 weeks? I am currently so busy to go update all my apps instantly.

[semistrict]

ok, I will leave the PR open and merge in two weeks.

[jeanbza]

Friendly ping - what's the timeline for this?

[rakyll]

Can we remove this now? goimports automatically picks up this package and confuses the user.

[odeke-em]

Please go ahead @Ramonza, sorry and thanks for the patient wait everyone!

[jcgregorio]

Please revert "Remove Stackdriver exporter" as it breaks google.golang.org/api/transport/http and thus a slew of cloud.google.com packages.

[jcgregorio]

Thanks!