Fix metric splitting edge cases (#2896)

Metrics needing one extra byte to fit the output buffer would not be split, so we would emit lines without a line ending. Metrics which overflowed by exactly one field length would be split one field too late, causing truncated fields.
(cherry picked from commit a275e6792a)

metric/metric.go

@@ -218,7 +218,7 @@ func (m *metric) SerializeTo(dst []byte) int {
 }
 
 func (m *metric) Split(maxSize int) []telegraf.Metric {
-	if m.Len() < maxSize {
+	if m.Len() <= maxSize {
 		return []telegraf.Metric{m}
 	}
 	var out []telegraf.Metric
@@ -248,7 +248,7 @@ func (m *metric) Split(maxSize int) []telegraf.Metric {
 
 		// if true, then we need to create a metric _not_ including the currently
 		// selected field
-		if len(m.fields[i:j])+len(fields)+constant > maxSize {
+		if len(m.fields[i:j])+len(fields)+constant >= maxSize {
 			// if false, then we'll create a metric including the currently
 			// selected field anyways. This means that the given maxSize is too
 			// small for a single field to fit.

metric/metric_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/influxdata/telegraf"
 
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 func TestNewMetric(t *testing.T) {
@@ -458,7 +459,7 @@ func TestSplitMetric(t *testing.T) {
 	assert.Len(t, split70, 3)
 
 	split60 := m.Split(60)
-	assert.Len(t, split60, 4)
+	assert.Len(t, split60, 5)
 }
 
 // test splitting metric into various max lengths
@@ -578,6 +579,42 @@ func TestSplitMetric_OneField(t *testing.T) {
 	assert.Equal(t, "cpu,host=localhost float=100001 1480940990034083306\n", split[0].String())
 }
 
+func TestSplitMetric_ExactSize(t *testing.T) {
+	now := time.Unix(0, 1480940990034083306)
+	tags := map[string]string{
+		"host": "localhost",
+	}
+	fields := map[string]interface{}{
+		"float":  float64(100001),
+		"int":    int64(100001),
+		"bool":   true,
+		"false":  false,
+		"string": "test",
+	}
+	m, err := New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+	actual := m.Split(m.Len())
+	// check that no copy was made
+	require.Equal(t, &m, &actual[0])
+}
+
+func TestSplitMetric_NoRoomForNewline(t *testing.T) {
+	now := time.Unix(0, 1480940990034083306)
+	tags := map[string]string{
+		"host": "localhost",
+	}
+	fields := map[string]interface{}{
+		"float": float64(100001),
+		"int":   int64(100001),
+		"bool":  true,
+		"false": false,
+	}
+	m, err := New("cpu", tags, fields, now)
+	assert.NoError(t, err)
+	actual := m.Split(m.Len() - 1)
+	require.Equal(t, 2, len(actual))
+}
+
 func TestNewMetricAggregate(t *testing.T) {
 	now := time.Now()