fix: label values regex for single letter labels

Closes: #13641

public/app/plugins/datasource/prometheus/metric_find_query.ts

@@ -12,7 +12,7 @@ export default class PrometheusMetricFindQuery {
   }
 
   process() {
-    const labelValuesRegex = /^label_values\((?:(.+),\s*)?([a-zA-Z_][a-zA-Z0-9_]+)\)\s*$/;
+    const labelValuesRegex = /^label_values\((?:(.+),\s*)?([a-zA-Z_][a-zA-Z0-9_]*)\)\s*$/;
     const metricNamesRegex = /^metrics\((.+)\)\s*$/;
     const queryResultRegex = /^query_result\((.+)\)\s*$/;