v3 refactor (#1868)

* Move to v3

Co-authored-by: Ben Toogood <bentoogood@gmail.com>

options.go

@@ -0,0 +1,18 @@
+// Package mdns provides a multicast dns registry
+package mdns
+
+import (
+	"context"
+
+	"github.com/micro/go-micro/v3/registry"
+)
+
+// Domain sets the mdnsDomain
+func Domain(d string) registry.Option {
+	return func(o *registry.Options) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, "mdns.domain", d)
+	}
+}