Upgrade etcd to 3.5

[sagikazarmark]

As of 3.5 etcd properly uses go modules.

[peterbourgon]

Hooray!

[sagikazarmark]

Sadly, it's still alpha though, hence the draft PR. But if you are okay with alpha versions, I can mark it as ready.

[peterbourgon]

@sagikazarmark Is 3.4 impossible?

[sagikazarmark]

@peterbourgon 3.4 won't receive proper go modules support. Furthermore, 3.5 introduces separate client modules which means less dependencies (we don't have to rely on the server stuff which probably has more deps).

[peterbourgon]

Issues keep piling up related to this dependency. Can we justifiably use https://github.com/etcd-io/etcd/releases/tag/v3.5.0-beta.3?

[sagikazarmark]

TBH I wouldn't recommend that. At least there were quite a few fixes to the release pipeline, so I'd probably wait until it's more stable.

The stable should be close IMHO.

[sagikazarmark]

Looks like the packaging issues were sorted out.

[peterbourgon]

I can't say I understand why all of these are here. Maybe a quick summary?

[sagikazarmark]

go.etcd.io/etcd/client/v2: used by sd/etcd (v2 client became a separate module)
go.etcd.io/etcd/client/v3: used by sd/etcdv3
go.etcd.io/etcd/client/pkg/v3: some common client code has been extracted to separate a pkg module (go-kit uses the transport package from there)

etcd decided to slice up the project into smaller modules.

Does that help?

[peterbourgon]

#1097