Fix panic if JSONNameKey is not found (#4735)

(cherry picked from commit 4c9c31c)

Author: glinton

plugins/parsers/json/README.md

@@ -108,7 +108,7 @@ Config:
   files = ["example"]
   name_key = "name"
   tag_keys = ["my_tag_1"]
-  string_fields = ["my_field"]
+  string_fields = ["b_my_field"]
   data_format = "json"
 ```
 
@@ -127,7 +127,7 @@ Input:
 
 Output:
 ```
-my_json,my_tag_1=foo a=5,b_c=6,my_field="description"
+my_json,my_tag_1=foo a=5,b_c=6,b_my_field="description"
 ```
 
 #### Arrays

plugins/parsers/json/parser.go

@@ -5,6 +5,8 @@ import (
 	"encoding/json"
 	"fmt"
 	"log"
+	"math"
+	"regexp"
 	"strconv"
 	"strings"
 	"time"
@@ -13,8 +15,6 @@ import (
 	"github.com/influxdata/telegraf/metric"
 	"github.com/pkg/errors"
 	"github.com/tidwall/gjson"
-	"math"
-	"regexp"
 )
 
 var (
@@ -94,7 +94,6 @@ func parseUnixTimestamp(jsonValue interface{}, format string) (time.Time, error)
 }
 
 func (p *JSONParser) parseObject(metrics []telegraf.Metric, jsonOut map[string]interface{}) ([]telegraf.Metric, error) {
-
 	tags := make(map[string]string)
 	for k, v := range p.DefaultTags {
 		tags[k] = v
@@ -108,7 +107,10 @@ func (p *JSONParser) parseObject(metrics []telegraf.Metric, jsonOut map[string]i
 
 	//checks if json_name_key is set
 	if p.JSONNameKey != "" {
-		p.MetricName = f.Fields[p.JSONNameKey].(string)
+		switch field := f.Fields[p.JSONNameKey].(type) {
+		case string:
+			p.MetricName = field
+		}
 	}
 
 	//if time key is specified, set it to nTime