Add docs for Grafana dashboard

content/docs/2.8/operate/prometheus.md

@@ -6,19 +6,44 @@ weight = 200
 
 ## Prometheus Exporter Metrics
 
-### Operator 
+### Operator
 
 The KEDA Operator exposes Prometheus metrics which can be scraped on port `8080` at `/metrics`. The following metrics are being gathered:
 
 - Metrics exposed by the `Operator SDK` framework as explained [here](https://sdk.operatorframework.io/docs/building-operators/golang/advanced-topics/#metrics).
 
 ### Metrics Adapter
 
-The KEDA Metrics Adapter exposes Prometheus metrics which can be scraped on port `9022` (this can be changed by setting the `metrics-port` argument for the Metrics Adapter) at `/metrics`.  The metrics collected in the Metrics Adapter are only active when the HPA is active (> 0 replicas).
+The KEDA Metrics Adapter exposes Prometheus metrics which can be scraped on port `9022` (this can be changed by setting the `metrics-port` argument for the Metrics Adapter) at `/metrics`. The metrics collected in the Metrics Adapter are only active when the HPA is active (> 0 replicas).
 
 The following metrics are being gathered:
 
 - `keda_metrics_adapter_scaler_error_totals` - The total number of errors encountered for all scalers.
 - `keda_metrics_adapter_scaled_object_error_totals`- The number of errors that have occurred for each scaled object.
 - `keda_metrics_adapter_scaler_errors` - The number of errors that have occurred for each scaler.
 - `keda_metrics_adapter_scaler_metrics_value`- The current value for each scaler's metric that would be used by the HPA in computing the target average.
+
+## Premade Grafana dashboard
+
+A premade [Grafana dashboard](https://github.com/kedacore/keda/tree/main/config/grafana/keda-dashboard.json) is available to visualize metrics exposed by the KEDA Metrics Adapter.
+
+![KEDA Grafana dashboard](/img/grafana-dashboard.png)
+
+The dashboard has two sections; the metric server visualizes metrics from KEDA, and the scale target visualizes the changes in replicas initially triggered by KEDA.
+
+The metric queries are:
+
+- `sum by(job) (rate(keda_metrics_adapter_scaler_errors_total{}[5m]))`
+- `sum by(scaler) (rate(keda_metrics_adapter_scaler_errors{namespace=~"$namespace", scaledObject=~"$scaledObject", scaler=~"$scaler"}[5m]))`
+- `sum by(scaledObject) (rate(keda_metrics_adapter_scaled_object_errors{namespace=~"$namespace", scaledObject=~"$scaledObject"}[5m]))`
+- `sum by(metric) (rate(keda_metrics_adapter_scaler_metrics_value{namespace=~"$namespace", metric=~"$metric"}[5m]))`
+- `changes(kube_deployment_spec_replicas{deployment=~"$scaledObject"}[1m])`
+- `changes(kube_statefulset_replicas{statefulset=~"$scaledObject"}[1m])`
+
+On top, the dashboard supports the following variables:
+
+- datasource
+- namespace
+- scaledObject
+- scaler
+- metric