multitenant: serialise report to buffer before sending

Seems to be faster

app/server_helpers.go

@@ -1,12 +1,14 @@
 package app
 
 import (
+	"bytes"
 	"context"
 	"net/http"
 	"strings"
 
 	opentracing "github.com/opentracing/opentracing-go"
 	"github.com/ugorji/go/codec"
+	"github.com/weaveworks/scope/report"
 
 	log "github.com/sirupsen/logrus"
 )
@@ -37,15 +39,20 @@ func respondWith(ctx context.Context, w http.ResponseWriter, code int, response
 
 // Similar to the above function, but respect the request's Accept header.
 // Possibly we should do a complete parse of Accept, but for now just rudimentary check
-func respondWithReport(ctx context.Context, w http.ResponseWriter, req *http.Request, response interface{}) {
+func respondWithReport(ctx context.Context, w http.ResponseWriter, req *http.Request, response report.Report) {
 	accept := req.Header.Get("Accept")
 	if strings.HasPrefix(accept, "application/msgpack") {
-		w.Header().Set("Content-Type", "application/msgpack")
-		w.WriteHeader(http.StatusOK)
-		encoder := codec.NewEncoder(w, &codec.MsgpackHandle{})
+		buf := bytes.Buffer{}
+		encoder := codec.NewEncoder(&buf, &codec.MsgpackHandle{})
 		if err := encoder.Encode(response); err != nil {
 			log.Errorf("Error encoding response: %v", err)
 		}
+		if span := opentracing.SpanFromContext(ctx); span != nil {
+			span.LogKV("encoded-size", len(buf.Bytes()))
+		}
+		w.Header().Set("Content-Type", "application/msgpack")
+		w.WriteHeader(http.StatusOK)
+		w.Write(buf.Bytes())
 		return
 	}