Possibility to extract parts of a json-message. (#271)

* Possibility to extract parts of a json-message.

* Fix some small things based on review comments.

* Fix review comment

Co-authored-by: Takashi Kusumi <tkusumi@zlab.co.jp>

---------

Co-authored-by: Takashi Kusumi <tkusumi@zlab.co.jp>

README.md

@@ -155,6 +155,8 @@ functions](https://golang.org/pkg/text/template/#hdr-Functions)):
 | `color`         | `color.Color, string` | Wrap the text in color (.ContainerColor and .PodColor provided)                   |
 | `parseJSON`     | `string`              | Parse string as JSON                                                              |
 | `tryParseJSON`  | `string`              | Attempt to parse string as JSON, return nil on failure                             |
+| `extractJSONParts`    | `string, ...string` | Parse string as JSON and concatenate the given keys.                          |
+| `tryExtractJSONParts` | `string, ...string` | Attempt to parse string as JSON and concatenate the given keys. , return text on failure |
 | `extjson`       | `string`              | Parse the object as json and output colorized json                                |
 | `ppextjson`     | `string`              | Parse the object as json and output pretty-print colorized json                   |
 | `toRFC3339Nano` | `object`              | Parse timestamp (string, int, json.Number) and output it using RFC3339Nano format |

cmd/cmd.go

@@ -450,6 +450,28 @@ func (o *options) generateTemplate() (*template.Template, error) {
 			}
 			return obj, nil
 		},
+		"extractJSONParts": func(text string, part ...string) (string, error) {
+			obj := make(map[string]interface{})
+			if err := json.Unmarshal([]byte(text), &obj); err != nil {
+				return "", err
+			}
+			parts := make([]string, 0)
+			for _, key := range part {
+				parts = append(parts, fmt.Sprintf("%v", obj[key]))
+			}
+			return strings.Join(parts, ", "), nil
+		},
+		"tryExtractJSONParts": func(text string, part ...string) string {
+			obj := make(map[string]interface{})
+			if err := json.Unmarshal([]byte(text), &obj); err != nil {
+				return text
+			}
+			parts := make([]string, 0)
+			for _, key := range part {
+				parts = append(parts, fmt.Sprintf("%v", obj[key]))
+			}
+			return strings.Join(parts, ", ")
+		},
 		"extjson": func(in string) (string, error) {
 			if json.Valid([]byte(in)) {
 				return strings.TrimSuffix(in, "\n"), nil