Fix selector / series formatting for empty metric names

Fixes https://github.com/prometheus/prometheus/issues/15335

Signed-off-by: Julius Volz <julius.volz@gmail.com>

web/ui/mantine-ui/src/promql/serializeAndFormat.test.ts

@@ -157,6 +157,20 @@ describe("serializeNode and formatNode", () => {
         },
         output: "metric_name[5m] @ start() offset -10m",
       },
+      {
+        node: {
+          type: nodeType.vectorSelector,
+          name: "", // Test formatting a selector with an empty metric name.
+          matchers: [
+            { type: matchType.equal, name: "label1", value: "value1" },
+          ],
+          offset: 0,
+          timestamp: null,
+          startOrEnd: null,
+        },
+        output:
+          '{label1="value1"}',
+      },
 
       // Aggregations.
       {

web/ui/mantine-ui/src/promql/utils.ts

@@ -271,7 +271,7 @@ const metricNameRe = /^[a-zA-Z_:][a-zA-Z0-9_:]*$/;
 const labelNameCharsetRe = /^[a-zA-Z_][a-zA-Z0-9_]*$/;
 
 export const metricContainsExtendedCharset = (str: string) => {
-  return !metricNameRe.test(str);
+  return str !== "" && !metricNameRe.test(str);
 };
 
 export const labelNameContainsExtendedCharset = (str: string) => {