Ignore UTF8 BOM in JSON parser (#4099)

plugins/parsers/json/parser.go

@@ -12,6 +12,10 @@ import (
 	"github.com/influxdata/telegraf/metric"
 )
 
+var (
+	utf8BOM = []byte("\xef\xbb\xbf")
+)
+
 type JSONParser struct {
 	MetricName  string
 	TagKeys     []string
@@ -68,6 +72,7 @@ func (p *JSONParser) parseObject(metrics []telegraf.Metric, jsonOut map[string]i
 
 func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
 	buf = bytes.TrimSpace(buf)
+	buf = bytes.TrimPrefix(buf, utf8BOM)
 	if len(buf) == 0 {
 		return make([]telegraf.Metric, 0), nil
 	}

plugins/parsers/json/parser_test.go

@@ -428,3 +428,15 @@ func TestParseArrayWithTagKeys(t *testing.T) {
 		"othertag": "baz",
 	}, metrics[1].Tags())
 }
+
+var jsonBOM = []byte("\xef\xbb\xbf[{\"value\":17}]")
+
+func TestHttpJsonBOM(t *testing.T) {
+	parser := JSONParser{
+		MetricName: "json_test",
+	}
+
+	// Most basic vanilla test
+	_, err := parser.Parse(jsonBOM)
+	assert.NoError(t, err)
+}