Allow numeric and non-string values for tag_keys. (#1782)

* Allow numeric and non-string values for tag_keys.

According to the go documentation the JSON deserializer only produces these
base types in output:
- string
- bool
- float64
- nil
With this patch bool, float64 and nil values get converted to a string when
their field key is specified in tag_keys. Previously the field was simply
discarded.

* Updated handling of nil for passing tests.

The automated tests are less than trivial to reproduece locally for me,
so I hope CircleCI wonn't mind...

* Updated changelog entries with PR and issue links.

plugins/parsers/json/parser.go

@@ -35,6 +35,10 @@ func (p *JSONParser) Parse(buf []byte) ([]telegraf.Metric, error) {
 		switch v := jsonOut[tag].(type) {
 		case string:
 			tags[tag] = v
+		case bool:
+			tags[tag] = strconv.FormatBool(v)
+		case float64:
+			tags[tag] = strconv.FormatFloat(v, 'f', -1, 64)
 		}
 		delete(jsonOut, tag)
 	}