Fix InfluxDB output UDP line splitting (#5439)

plugins/serializers/influx/influx.go

@@ -240,6 +240,7 @@ func (s *Serializer) writeMetric(w io.Writer, m telegraf.Metric) error {
 				return err
 			}
 
+			pairsLen = 0
 			firstField = true
 			bytesNeeded = len(s.header) + len(s.pair) + len(s.footer)
 

plugins/serializers/influx/influx_test.go

@@ -275,6 +275,24 @@ var tests = []struct {
 		),
 		output: []byte("cpu abc=123i 1519194109000000042\ncpu def=456i 1519194109000000042\n"),
 	},
+	{
+		name:     "split_fields_overflow",
+		maxBytes: 43,
+		input: MustMetric(
+			metric.New(
+				"cpu",
+				map[string]string{},
+				map[string]interface{}{
+					"abc": 123,
+					"def": 456,
+					"ghi": 789,
+					"jkl": 123,
+				},
+				time.Unix(1519194109, 42),
+			),
+		),
+		output: []byte("cpu abc=123i,def=456i 1519194109000000042\ncpu ghi=789i,jkl=123i 1519194109000000042\n"),
+	},
 	{
 		name: "name newline",
 		input: MustMetric(