running output: Drop nil metrics

fixes #2317

internal/models/running_output.go

@@ -90,6 +90,9 @@ func NewRunningOutput(
 // AddMetric adds a metric to the output. This function can also write cached
 // points if FlushBufferWhenFull is true.
 func (ro *RunningOutput) AddMetric(m telegraf.Metric) {
+	if m == nil {
+		return
+	}
 	// Filter any tagexclude/taginclude parameters before adding metric
 	if ro.Config.Filter.IsActive() {
 		// In order to filter out tags, we need to create a new metric, since

internal/models/running_output_test.go

@@ -75,6 +75,23 @@ func BenchmarkRunningOutputAddFailWrites(b *testing.B) {
 	}
 }
 
+func TestAddingNilMetric(t *testing.T) {
+	conf := &OutputConfig{
+		Filter: Filter{},
+	}
+
+	m := &mockOutput{}
+	ro := NewRunningOutput("test", m, conf, 1000, 10000)
+
+	ro.AddMetric(nil)
+	ro.AddMetric(nil)
+	ro.AddMetric(nil)
+
+	err := ro.Write()
+	assert.NoError(t, err)
+	assert.Len(t, m.Metrics(), 0)
+}
+
 // Test that NameDrop filters ger properly applied.
 func TestRunningOutput_DropFilter(t *testing.T) {
 	conf := &OutputConfig{