clientv3: embedded client

[heyitsanthony]

In the future, some features implemented through the client may be "embedded" in the server for better performance (e.g., locks). Ideally, these features should work on the server without additional modification. Right now clientv3 expects to connect via gRPC to an etcd v3 server. In order to get good performance for the embedded case, the embedded client implementation should directly call the interfaces in api/v3rpc instead of going through grpc.

[timothysc]

/cc @smarterclayton fyi.

[heyitsanthony]

done in #7444 via etcdserver/api/v3client

[xiang90]

@heyitsanthony Do we have an example or doc to talk about how to embed client?

[heyitsanthony]

@xiang90 no. Is this a feature that should be advertised with examples or a doc? I think the usage is kind of straightforward, but an example could be added to v3client's doc.go...

[xiang90]

@heyitsanthony Initially an example would be good. If people start to ask about it, we can add a doc. What do you think?

[heyitsanthony]

ok will add something to doc.go

[timothysc]

xref: kubernetes/kubernetes#43088