Prometheus adapter custom time range (#1033)

Update Developer doc with customize interval explanation. Add example of
VerticaAutoscaler under sample.

DEVELOPER.md

@@ -591,6 +591,7 @@ A Prometheus service is running and accessible through an URL.
 
 2. Configuration
    Customize the adapter configuration to map Prometheus metrics to Kubernetes metrics. For example, you can configure rules to map a Prometheus query (e.g., vertica_query_requests_attempted_total) to a Kubernetes metric.
+   To customize the interval of a query, change the interval value "5m" on the metricsQuery to the prefered time range.
 
    ```shell
    rules:

config/samples/v1beta1_verticaautoscaler_custom_metrics.yaml

@@ -0,0 +1,22 @@
+apiVersion: vertica.com/v1beta1
+kind: VerticaAutoscaler
+metadata:
+  name: as-sample-custom-metrics
+spec:
+  verticaDBName: verticadb-sample
+  # The serviceName should be the name of your subcluster
+  serviceName: sc1
+  scalingGranularity: Pod
+  customAutoscalerSpec:
+    minReplicas: 3
+    maxReplicas: 10
+    metrics:    
+    - metric:
+        type: Pods
+        pods:
+          metric:
+            # The name of the mapped Kubernetes metric name
+            name: vertica_query_requests_attempted_rate_per_second
+          target:
+            type: AverageValue
+            averageValue: 20000m