Allow metrics to be unserializable in influx.Reader (#4047)

Metrics that are unserializable will be logged at debug level, but the
rest of the batch will be sent.  Unserializable metrics can occur during
normal operation such as if you remove all fields from a metric or the
metric cannot fit within the line size limit.

plugins/serializers/influx/reader.go

@@ -2,7 +2,9 @@ package influx
 
 import (
 	"bytes"
+	"fmt"
 	"io"
+	"log"
 
 	"github.com/influxdata/telegraf"
 )
@@ -47,11 +49,25 @@ func (r *reader) Read(p []byte) (int, error) {
 		return 0, io.EOF
 	}
 
-	_, err := r.serializer.Write(r.buf, r.metrics[r.offset])
-	r.offset += 1
-	if err != nil {
-		r.buf.Reset()
-		return 0, err
+	for _, metric := range r.metrics[r.offset:] {
+		_, err := r.serializer.Write(r.buf, metric)
+		r.offset += 1
+		if err != nil {
+			r.buf.Reset()
+			switch err.(type) {
+			case *MetricError:
+				// Since we are serializing an array of metrics, don't fail
+				// the entire batch just because of one unserializable metric.
+				log.Printf(
+					"D! [serializers.influx] could not serialize metric %q: %v; discarding metric",
+					metric.Name(), err)
+				continue
+			default:
+				fmt.Println(err)
+				return 0, err
+			}
+		}
+		break
 	}
 
 	return r.buf.Read(p)