mysql: convert numbers to text for annotation tooltip

public/app/plugins/datasource/mysql/response_parser.ts

@@ -138,7 +138,7 @@ export default class ResponseParser {
       list.push({
         annotation: options.annotation,
         time: Math.floor(row[timeColumnIndex]) * 1000,
-        text: row[textColumnIndex],
+        text: row[textColumnIndex] ? row[textColumnIndex].toString() : '',
         tags: row[tagsColumnIndex] ? row[tagsColumnIndex].trim().split(/\s*,\s*/) : [],
       });
     }