client/selector: query across multiple domains (#1725)

* client/selector: query across multiple domains

* client/selector: check for nil services

* config/cmd: fix merge bug

client/selector/options.go

@@ -10,6 +10,9 @@ type Options struct {
 	Registry registry.Registry
 	Strategy Strategy
 
+	// Domain to lookup services from within the registry
+	Domain string
+
 	// Other options for implementations of the interface
 	// can be stored in a context
 	Context context.Context
@@ -37,6 +40,13 @@ func Registry(r registry.Registry) Option {
 	}
 }
 
+// Domain sets the domain used by the selector
+func Domain(d string) Option {
+	return func(o *Options) {
+		o.Domain = d
+	}
+}
+
 // SetStrategy sets the default strategy for the selector
 func SetStrategy(fn Strategy) Option {
 	return func(o *Options) {