fixup message options and content-type header

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

kgo.go

@@ -156,7 +156,7 @@ func (b *Broker) NewMessage(ctx context.Context, hdr metadata.Metadata, body int
 		options.ContentType = b.opts.ContentType
 	}
 
-	m := &kgoMessage{ctx: ctx, hdr: hdr, opts: options}
+	m := &kgoMessage{ctx: ctx, hdr: hdr.Copy(), opts: options}
 	c, err := b.newCodec(m.opts.ContentType)
 	if err == nil {
 		m.body, err = c.Marshal(body)
@@ -165,6 +165,8 @@ func (b *Broker) NewMessage(ctx context.Context, hdr metadata.Metadata, body int
 		return nil, err
 	}
 
+	m.hdr.Set(metadata.HeaderContentType, m.opts.ContentType)
+
 	return m, nil
 }
 
@@ -364,6 +366,19 @@ func (b *Broker) publish(ctx context.Context, topic string, messages ...broker.M
 			}
 		}
 
+		kmsg, ok := msg.(*kgoMessage)
+		if !ok {
+			continue
+		}
+		if kmsg.opts.Context != nil {
+			if k, ok := kmsg.opts.Context.Value(messageKey{}).([]byte); ok && k != nil {
+				rec.Key = k
+			}
+			if p, ok := kmsg.opts.Context.Value(messagePromiseKey{}).(func(*kgo.Record, error)); ok && p != nil {
+				promise = p
+			}
+		}
+
 		setHeaders(rec, msg.Header())
 
 		if promise != nil {