func Set(addr resolver.Address, md metadata.MD) resolver.Address {
	addr.Attributes = addr.Attributes.WithValues(mdKey, md)
	return addr
}
Set sets (overrides) the metadata in addr.

When a SubConn is created with this address, the RPCs sent on it will all
have this metadata.
package metadata
import (
	"google.golang.org/grpc/metadata"
	"google.golang.org/grpc/resolver"
)
type mdKeyType string
const mdKey = mdKeyType("grpc.internal.address.metadata")
func Get(addr resolver.Address) metadata.MD {
	attrs := addr.Attributes
	if attrs == nil {
		return nil
	}
	md, _ := attrs.Value(mdKey).(metadata.MD)
	return md
}