Allow metrics to be defined as namespaced: false

When set to false, no namespace label will be set by the adapter based on the namespace
portion of the url in the request path.

This allows individual consumers to set namespace independent of the source kubernetes resource.

---

Example:

Given an adapter config like this:

```
    externalRules:
    - seriesQuery: 'nsq_topic_depth'
      resources:
        namespaced: false
```

An HPA could target a different namespace by setting it in the selector:

```
  - type: External
    external:
      metric:
        name: nsq_topic_depth
        selector:
          labelSelector:
            topic: my-topic
            namespace: nsq
```

This is useful for scaling on metrics from services that run in a differnt namespace than the source resource.

pkg/naming/metric_namer.go

@@ -102,10 +102,15 @@ type metricNamer struct {
 	nameMatches    *regexp.Regexp
 	nameAs         string
 	seriesMatchers []*ReMatcher
+	namespaced     *bool
 
 	ResourceConverter
 }
 
+func (n *metricNamer) isNamespaced() bool {
+	return (n.namespaced != nil) && *n.namespaced
+}
+
 // queryTemplateArgs are the arguments for the metrics query template.
 func (n *metricNamer) FilterSeries(initialSeries []prom.Series) []prom.Series {
 	if len(n.seriesMatchers) == 0 {
@@ -131,8 +136,10 @@ func (n *metricNamer) QueryForSeries(series string, resource schema.GroupResourc
 }
 
 func (n *metricNamer) QueryForExternalSeries(series string, namespace string, metricSelector labels.Selector) (prom.Selector, error) {
-	//test := prom.Selector()
-	//return test, nil
+	if !n.isNamespaced() {
+		namespace = ""
+	}
+
 	return n.metricsQuery.BuildExternal(series, namespace, "", []string{}, metricSelector)
 }
 
@@ -207,6 +214,7 @@ func NamersFromConfig(cfg []config.DiscoveryRule, mapper apimeta.RESTMapper) ([]
 			nameMatches:       nameMatches,
 			nameAs:            nameAs,
 			seriesMatchers:    seriesMatchers,
+			namespaced:        rule.Resources.Namespaced,
 			ResourceConverter: resConv,
 		}