allocations improvements and tunnel fixes (#1248)

* reduce allocations in tunnel code

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

* another allocation fix

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

* allocate maps with len if it known

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

* allocate key for send once

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

grpc.go

@@ -110,13 +110,18 @@ func (g *grpcClient) next(request client.Request, opts client.CallOptions) (sele
 }
 
 func (g *grpcClient) call(ctx context.Context, node *registry.Node, req client.Request, rsp interface{}, opts client.CallOptions) error {
+	var header map[string]string
+
 	address := node.Address
 
-	header := make(map[string]string)
+	header = make(map[string]string)
 	if md, ok := metadata.FromContext(ctx); ok {
+		header = make(map[string]string, len(md))
 		for k, v := range md {
 			header[k] = v
 		}
+	} else {
+		header = make(map[string]string)
 	}
 
 	// set timeout in nanoseconds
@@ -182,13 +187,17 @@ func (g *grpcClient) call(ctx context.Context, node *registry.Node, req client.R
 }
 
 func (g *grpcClient) stream(ctx context.Context, node *registry.Node, req client.Request, opts client.CallOptions) (client.Stream, error) {
+	var header map[string]string
+
 	address := node.Address
 
-	header := make(map[string]string)
 	if md, ok := metadata.FromContext(ctx); ok {
+		header = make(map[string]string, len(md))
 		for k, v := range md {
 			header[k] = v
 		}
+	} else {
+		header = make(map[string]string)
 	}
 
 	// set timeout in nanoseconds