func WithProxy(ctx context.Context, addrs ...string) context.Context {
	if md, ok := metadata.FromContext(ctx); ok {
		return metadata.NewOutgoingContext(ctx, metadata.MD{
			proxyKey: append(md[proxyKey], addrs...),
		})
	}
	return metadata.NewOutgoingContext(ctx, metadata.MD{proxyKey: addrs})
}
WithProxy - Sets proxies in context metadata.
package crawl
import (
	"golang.org/x/net/context"
	"google.golang.org/grpc/metadata"
)
var proxyKey = "crawl_proxy"