admin: fix nil panic when retrieving metric metadata

Previously, the code that retrieved metric metadata assumed that the underlying
node would have at least a single store configured in the `MetricRecorder`
instance. This is not true on a SQL tenant since the storage node abstracts
away all store-related information.

The code now checks to make sure that at least one store is found before moving
ahead with retrieving store metric metadata.

Epic: CRDB-12100
Release note: None

pkg/ccl/serverccl/adminccl/tenant_admin_test.go

@@ -48,6 +48,19 @@ func TestTenantAdminAPI(t *testing.T) {
 	t.Run("tenant_unimplemented", func(t *testing.T) {
 		testUnimplementedRPCs(ctx, t, testHelper)
 	})
+
+	t.Run("tenant_metricmetadata", func(t *testing.T) {
+		testMetricMetadataRPC(ctx, t, testHelper)
+	})
+}
+
+func testMetricMetadataRPC(ctx context.Context, t *testing.T, helper serverccl.TenantTestHelper) {
+	http := helper.TestCluster().TenantAdminHTTPClient(t, 1)
+	defer http.Close()
+
+	metricMetadataResp := serverpb.MetricMetadataResponse{}
+	http.GetJSON("/_admin/v1/metricmetadata", &metricMetadataResp)
+	require.NotEmpty(t, metricMetadataResp.Metadata)
 }
 
 func testUnimplementedRPCs(ctx context.Context, t *testing.T, helper serverccl.TenantTestHelper) {

pkg/server/status/recorder.go

@@ -359,13 +359,17 @@ func (mr *MetricsRecorder) GetMetricsMetadata() map[string]metric.Metadata {
 	// Get a random storeID.
 	var sID roachpb.StoreID
 
+	storeFound := false
 	for storeID := range mr.mu.storeRegistries {
 		sID = storeID
+		storeFound = true
 		break
 	}
 
 	// Get metric metadata from that store because all stores have the same metadata.
-	mr.mu.storeRegistries[sID].WriteMetricsMetadata(metrics)
+	if storeFound {
+		mr.mu.storeRegistries[sID].WriteMetricsMetadata(metrics)
+	}
 
 	return metrics
 }