include add'l metadata in resolved addresses

[jhump]

This allows for implementing custom balancers that might be interested in additional information about the target endpoints.

This adds the endpoint reference (typically a pod reference) and its labels to the Attributes field of the resulting resolver.Address values.

[jhump]

@sercand, any chance you'd accept a small patch?

[sercand]

@jhump Thanks for the PR

[jhump]

@sercand, looks like this can tickle issues in gRPC, where typical balancer implementations will churn connections (e.g. close all connections and re-establish all new ones). I've filed grpc/grpc-go#3611.

Not sure if you want to revert this PR until that is remedied or just hold off on a release. But I figured I should let you know. (This change worked great in test environments; but it just hit our production environment yesterday, where the scale of connection churn started causing some issues 😦).

[lvyijin]

Hi，@sercand, we encountered the same problem as Jhump. And I would like to konw why changed Metadata interface{} in v1.19.1 to Attributes *attributes.Attributes in v1.26.0.

[sercand]

I just reverted the changes. I haven't upgraded our gRPC version to the latest that's why I missed the problem. I will look for this as soon as possible.