feat: add ability to change the key names of the message output

lib/LogMessage.js

@@ -164,14 +164,16 @@ class LogMessage {
 
   /**
    * The full log object. This is the object used in logMessage.toJSON() and when the log is written to the console.
-   * See [Log Output](#log-output) for more information.
    * @returns {Object} The full log object.
-   * @example
-   * log.info('This is some info').value;
-   * //=> { _logLevel: 'info', msg: 'This is some info', ... }
    */
   get value() {
-    return { _logLevel: this.level, msg: this.msg, ...this.meta, _tags: this.tags };
+    const opts = this[symbols.OPTS];
+    return {
+      [opts.levelKey]: opts.levelKey ? this.level : undefined,
+      [opts.messageKey]: this.msg,
+      ...this.meta,
+      [opts.tagsKey]: opts.tagsKey ? this.tags : undefined
+    };
   }
 
   /**