Echo: Don't notify if PerformanceBackend errors when flushing (#89379)

public/app/core/services/echo/backends/PerformanceBackend.ts

@@ -32,8 +32,16 @@ export class PerformanceBackend implements EchoBackend<PerformanceEvent, Perform
       return;
     }
 
-    backendSrv.post('/api/frontend-metrics', {
+    backendSrv
+      .post(
+        '/api/frontend-metrics',
+        {
           events: this.buffer,
+        },
+        { showErrorAlert: false }
+      )
+      .catch(() => {
+        // Just swallow this error - it's non-critical
       });
 
     this.buffer = [];