client: add call option helper

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

client/context.go

@@ -32,3 +32,13 @@ func SetPublishOption(k, v interface{}) PublishOption {
 		o.Context = context.WithValue(o.Context, k, v)
 	}
 }
+
+// SetCallOption returns a function to setup a context with given value
+func SetCallOption(k, v interface{}) CallOption {
+	return func(o *CallOptions) {
+		if o.Context == nil {
+			o.Context = context.Background()
+		}
+		o.Context = context.WithValue(o.Context, k, v)
+	}
+}