Lock buffer when adding metrics (#4514)

This function is not thread-safe but is currently used by multiple
goroutines in RunningOutput

internal/buffer/buffer.go

@@ -40,19 +40,19 @@ func (b *Buffer) Len() int {
 
 // Add adds metrics to the buffer.
 func (b *Buffer) Add(metrics ...telegraf.Metric) {
+	b.mu.Lock()
 	for i, _ := range metrics {
 		MetricsWritten.Incr(1)
 		select {
 		case b.buf <- metrics[i]:
 		default:
-			b.mu.Lock()
 			MetricsDropped.Incr(1)
 			<-b.buf
 			b.buf <- metrics[i]
+		}
+	}
 	b.mu.Unlock()
 }
-	}
-}
 
 // Batch returns a batch of metrics of size batchSize.
 // the batch will be of maximum length batchSize. It can be less than batchSize,