diff --git a/pkg/kapis/monitoring/v1alpha3/register.go b/pkg/kapis/monitoring/v1alpha3/register.go
index f1e778ce3..e180cb2ce 100644
--- a/pkg/kapis/monitoring/v1alpha3/register.go
+++ b/pkg/kapis/monitoring/v1alpha3/register.go
@@ -464,6 +464,38 @@ func AddToContainer(c *restful.Container, k8sClient kubernetes.Interface, monito
 		Returns(http.StatusOK, respOK, monitoring.Metric{})).
 		Produces(restful.MIME_JSON)
 
+	ws.Route(ws.GET("/targets/metadata").
+		To(h.handleMetadataQuery).
+		Doc("Get metadata of metrics in the whole cluster.").
+		Metadata(restfulspec.KeyOpenAPITags, []string{constants.CustomMetricsTag}).
+		Writes(model.Metadata{}).
+		Returns(http.StatusOK, respOK, model.Metadata{})).
+		Produces(restful.MIME_JSON)
+
+	ws.Route(ws.GET("/targets/labelsets").
+		To(h.handleMetricLabelSetQuery).
+		Doc("List all available labels and values of a metric within a specific time span in the whole cluster.").
+		Param(ws.QueryParameter("metric", "The name of the metric").DataType("string").Required(true)).
+		Param(ws.QueryParameter("start", "Start time of query. It is a string with Unix time format, eg. 1559347200. ").DataType("string").Required(true)).
+		Param(ws.QueryParameter("end", "End time of query. It is a string with Unix time format, eg. 1561939200. ").DataType("string").Required(true)).
+		Metadata(restfulspec.KeyOpenAPITags, []string{constants.CustomMetricsTag}).
+		Writes(model.MetricLabelSet{}).
+		Returns(http.StatusOK, respOK, model.MetricLabelSet{})).
+		Produces(restful.MIME_JSON)
+
+	ws.Route(ws.GET("/targets/query").
+		To(h.handleAdhocQuery).
+		Doc("Make an ad-hoc query in the whole cluster.").
+		Param(ws.QueryParameter("expr", "The expression to be evaluated.").DataType("string").Required(false)).
+		Param(ws.QueryParameter("start", "Start time of query. Use **start** and **end** to retrieve metric data over a time span. It is a string with Unix time format, eg. 1559347200. ").DataType("string").Required(false)).
+		Param(ws.QueryParameter("end", "End time of query. Use **start** and **end** to retrieve metric data over a time span. It is a string with Unix time format, eg. 1561939200. ").DataType("string").Required(false)).
+		Param(ws.QueryParameter("step", "Time interval. Retrieve metric data at a fixed interval within the time range of start and end. It requires both **start** and **end** are provided. The format is [0-9]+[smhdwy]. Defaults to 10m (i.e. 10 min).").DataType("string").DefaultValue("10m").Required(false)).
+		Param(ws.QueryParameter("time", "A timestamp in Unix time format. Retrieve metric data at a single point in time. Defaults to now. Time and the combination of start, end, step are mutually exclusive.").DataType("string").Required(false)).
+		Metadata(restfulspec.KeyOpenAPITags, []string{constants.CustomMetricsTag}).
+		Writes(monitoring.Metric{}).
+		Returns(http.StatusOK, respOK, monitoring.Metric{})).
+		Produces(restful.MIME_JSON)
+
 	c.Add(ws)
 	return nil
 }
diff --git a/pkg/models/monitoring/monitoring.go b/pkg/models/monitoring/monitoring.go
index b945a6c15..5fc49bea0 100644
--- a/pkg/models/monitoring/monitoring.go
+++ b/pkg/models/monitoring/monitoring.go
@@ -62,25 +62,31 @@ func NewMonitoringOperator(client monitoring.Interface, k8s kubernetes.Interface
 }
 
 func (mo monitoringOperator) GetMetric(expr, namespace string, time time.Time) (monitoring.Metric, error) {
-	// Different monitoring backend implementations have different ways to enforce namespace isolation.
-	// Each implementation should register itself to `ReplaceNamespaceFns` during init().
-	// We hard code "prometheus" here because we only support this datasource so far.
-	// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
-	expr, err := expressions.ReplaceNamespaceFns["prometheus"](expr, namespace)
-	if err != nil {
-		return monitoring.Metric{}, err
+	if namespace != "" {
+		// Different monitoring backend implementations have different ways to enforce namespace isolation.
+		// Each implementation should register itself to `ReplaceNamespaceFns` during init().
+		// We hard code "prometheus" here because we only support this datasource so far.
+		// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
+		var err error
+		expr, err = expressions.ReplaceNamespaceFns["prometheus"](expr, namespace)
+		if err != nil {
+			return monitoring.Metric{}, err
+		}
 	}
 	return mo.c.GetMetric(expr, time), nil
 }
 
 func (mo monitoringOperator) GetMetricOverTime(expr, namespace string, start, end time.Time, step time.Duration) (monitoring.Metric, error) {
-	// Different monitoring backend implementations have different ways to enforce namespace isolation.
-	// Each implementation should register itself to `ReplaceNamespaceFns` during init().
-	// We hard code "prometheus" here because we only support this datasource so far.
-	// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
-	expr, err := expressions.ReplaceNamespaceFns["prometheus"](expr, namespace)
-	if err != nil {
-		return monitoring.Metric{}, err
+	if namespace != "" {
+		// Different monitoring backend implementations have different ways to enforce namespace isolation.
+		// Each implementation should register itself to `ReplaceNamespaceFns` during init().
+		// We hard code "prometheus" here because we only support this datasource so far.
+		// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
+		var err error
+		expr, err = expressions.ReplaceNamespaceFns["prometheus"](expr, namespace)
+		if err != nil {
+			return monitoring.Metric{}, err
+		}
 	}
 	return mo.c.GetMetricOverTime(expr, start, end, step), nil
 }
@@ -101,14 +107,18 @@ func (mo monitoringOperator) GetMetadata(namespace string) Metadata {
 }
 
 func (mo monitoringOperator) GetMetricLabelSet(metric, namespace string, start, end time.Time) MetricLabelSet {
-	// Different monitoring backend implementations have different ways to enforce namespace isolation.
-	// Each implementation should register itself to `ReplaceNamespaceFns` during init().
-	// We hard code "prometheus" here because we only support this datasource so far.
-	// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
-	expr, err := expressions.ReplaceNamespaceFns["prometheus"](metric, namespace)
-	if err != nil {
-		klog.Error(err)
-		return MetricLabelSet{}
+	var expr = metric
+	var err error
+	if namespace != "" {
+		// Different monitoring backend implementations have different ways to enforce namespace isolation.
+		// Each implementation should register itself to `ReplaceNamespaceFns` during init().
+		// We hard code "prometheus" here because we only support this datasource so far.
+		// In the future, maybe the value should be returned from a method like `mo.c.GetMonitoringServiceName()`.
+		expr, err = expressions.ReplaceNamespaceFns["prometheus"](metric, namespace)
+		if err != nil {
+			klog.Error(err)
+			return MetricLabelSet{}
+		}
 	}
 	data := mo.c.GetMetricLabelSet(expr, start, end)
 	return MetricLabelSet{Data: data}
diff --git a/pkg/simple/client/monitoring/prometheus/prometheus.go b/pkg/simple/client/monitoring/prometheus/prometheus.go
index ccdfe11c3..f03e69647 100644
--- a/pkg/simple/client/monitoring/prometheus/prometheus.go
+++ b/pkg/simple/client/monitoring/prometheus/prometheus.go
@@ -147,9 +147,12 @@ func (p prometheus) GetNamedMetricsOverTime(metrics []string, start, end time.Ti
 
 func (p prometheus) GetMetadata(namespace string) []monitoring.Metadata {
 	var meta []monitoring.Metadata
+	var matchTarget string
 
-	// Filter metrics available to members of this namespace
-	matchTarget := fmt.Sprintf("{namespace=\"%s\"}", namespace)
+	if namespace != "" {
+		// Filter metrics available to members of this namespace
+		matchTarget = fmt.Sprintf("{namespace=\"%s\"}", namespace)
+	}
 	items, err := p.client.TargetsMetadata(context.Background(), matchTarget, "", "")
 	if err != nil {
 		klog.Error(err)