m3db writetagged应该并发做，不然会导致transfer rpc变慢 (#514)

* m3db writetagged应该并发做，不然会导致transfer rpc变慢

* go func指针传参问题

src/modules/transfer/backend/m3db/m3db.go

@@ -3,6 +3,7 @@ package m3db
 import (
 	"fmt"
 	"sync"
+	"sync/atomic"
 	"time"
 
 	"github.com/didi/nightingale/src/common/dataobj"
@@ -89,23 +90,31 @@ func (p *Client) Push2Queue(items []*dataobj.MetricValue) {
 		logger.Errorf("unable to get m3db session: %s", err)
 		return
 	}
-
-	errCnt := 0
+	var errCnt int32
+	var (
+		wg sync.WaitGroup
+	)
 	for _, item := range items {
-		if err := session.WriteTagged(
+		wg.Add(1)
+		go func(dm *dataobj.MetricValue) {
+			err := session.WriteTagged(
 				p.namespaceID,
-			mvID(item),
-			ident.NewTagsIterator(mvTags(item)),
-			time.Unix(item.Timestamp, 0),
-			item.Value,
+				mvID(dm),
+				ident.NewTagsIterator(mvTags(dm)),
+				time.Unix(dm.Timestamp, 0),
+				dm.Value,
 				xtime.Second,
-			nil,
-		); err != nil {
+				nil)
+			if err != nil {
 				logger.Errorf("unable to writeTagged: %s", err)
-			errCnt++
+				atomic.AddInt32(&errCnt, 1)
 			}
+			wg.Done()
+		}(item)
+
 	}
-	stats.Counter.Set("m3db.queue.err", errCnt)
+	wg.Wait()
+	stats.Counter.Set("m3db.queue.err", int(errCnt))
 }
 
 // QueryData: || (|| endpoints...) (&& tags...)