Add gRPC render metadata header

In headers, we now send the number of metrics gRPC render will
stream. This can be beneficial for the receiver to know how many
metrics it will receive over the stream.

carbonserver/render.go

@@ -7,6 +7,7 @@ import (
 	"encoding/hex"
 	"encoding/json"
 	"fmt"
+	"google.golang.org/grpc/metadata"
 	"io"
 	"math"
 	"net/http"
@@ -646,6 +647,13 @@ func (listener *CarbonserverListener) streamMetrics(stream grpcv2.CarbonV2_Rende
 	return
 }
 
+const gRPCRenderMetricsCountHeaderKey = "metrics-count"
+
+func (listener *CarbonserverListener) sendRenderMetadataHeader(stream grpcv2.CarbonV2_RenderServer, filesCount int) error {
+	header := metadata.Pairs(gRPCRenderMetricsCountHeaderKey, strconv.Itoa(filesCount))
+	return stream.SendHeader(header)
+}
+
 // Render implements Render rpc of CarbonV2 gRPC service
 func (listener *CarbonserverListener) Render(req *protov2.MultiFetchRequest, stream grpcv2.CarbonV2_RenderServer) (rpcErr error) {
 	t0 := time.Now()
@@ -726,6 +734,10 @@ func (listener *CarbonserverListener) Render(req *protov2.MultiFetchRequest, str
 		metricGlobMap := getMetricGlobMapFromExpandedGlobs(expandedGlobs)
 		tle.MetricGlobMapLength = len(metricGlobMap)
 		filesCount := countFilesInExpandedGlobs(expandedGlobs)
+		err = listener.sendRenderMetadataHeader(stream, filesCount)
+		if err != nil {
+			return nil, err
+		}
 		prepareChan := make(chan response, getStreamingChannelSize(filesCount))
 		go func() {
 			prepareT0 := time.Now()
@@ -768,6 +780,7 @@ func (listener *CarbonserverListener) Render(req *protov2.MultiFetchRequest, str
 		case res != nil:
 			atomic.AddUint64(&listener.metrics.QueryCacheHit, 1)
 			cachedResponses := res.([]response)
+			err = listener.sendRenderMetadataHeader(stream, len(cachedResponses))
 			responseChanToStream = make(chan response, getStreamingChannelSize(len(cachedResponses)))
 			go func() {
 				for _, r := range cachedResponses {