add compile-time interface compliance check

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

message.go

@@ -0,0 +1,44 @@
+package http
+
+import (
+	"go.unistack.org/micro/v3/client"
+	"go.unistack.org/micro/v3/metadata"
+)
+
+type httpEvent struct {
+	payload     interface{}
+	topic       string
+	contentType string
+	opts        client.MessageOptions
+}
+
+func newHTTPEvent(topic string, payload interface{}, contentType string, opts ...client.MessageOption) client.Message {
+	options := client.NewMessageOptions(opts...)
+
+	if len(options.ContentType) > 0 {
+		contentType = options.ContentType
+	}
+
+	return &httpEvent{
+		payload:     payload,
+		topic:       topic,
+		contentType: contentType,
+		opts:        options,
+	}
+}
+
+func (c *httpEvent) ContentType() string {
+	return c.contentType
+}
+
+func (c *httpEvent) Topic() string {
+	return c.topic
+}
+
+func (c *httpEvent) Payload() interface{} {
+	return c.payload
+}
+
+func (c *httpEvent) Metadata() metadata.Metadata {
+	return c.opts.Metadata
+}