Add datadog metric provider to docs

Ref: #460

docs/gitbook/usage/metrics.md

@@ -5,7 +5,7 @@ availability, error rate percentage, average response time and any other objecti
 If a drop in performance is noticed during the SLOs analysis,
 the release will be automatically rolled back with minimum impact to end-users.
 
-### Builtin Metrics
+### Builtin metrics
 
 Flagger comes with two builtin metric checks: HTTP request success rate and duration.
 
@@ -31,12 +31,59 @@ and the window size or the time series with `interval`.
 The builtin checks are available for every service mesh / ingress controller
 and are implemented with [Prometheus queries](../faq.md#metrics).
 
-### Custom Metrics
+### Custom metrics
 
 The canary analysis can be extended with custom metric checks. Using a `MetricTemplate` custom resource, you 
 configure Flagger to connect to a metric provider and run a query that returns a `float64` value.
 The query result is used to validate the canary based on the specified threshold range.
 
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: MetricTemplate
+metadata:
+ name: my-metric
+spec:
+ provider:
+ type: # can be prometheus or datadog
+ address: # API URL
+ secretRef:
+ name: # name of the secret containing the API credentials
+ query: # metric query
+```
+
+The following variables are available in query templates:
+
+- `name` (canary.metadata.name)
+- `namespace` (canary.metadata.namespace)
+- `target` (canary.spec.targetRef.name)
+- `service` (canary.spec.service.name)
+- `ingress` (canary.spec.ingresRef.name)
+- `interval` (canary.spec.canaryAnalysis.metrics[].interval)
+
+A canary analysis metric can reference a template with `templateRef`:
+
+```yaml
+ canaryAnalysis:
+ metrics:
+ - name: "my metric"
+ templateRef:
+ name: my-metric
+ # namespace is optional
+ # when not specified, the canary namespace will be used
+ namespace: flagger
+ # accepted values
+ thresholdRange:
+ min: 10
+ max: 1000
+ # metric query time window
+ interval: 1m
+```
+
+### Prometheus 
+
+You can create custom metric checks targeting a Prometheus server
+by setting the provider type to `prometheus` and writing the query in PromQL.
+
 Prometheus template example:
 
 ```yaml
@@ -72,25 +119,14 @@ spec:
  ) * 100
 ```
 
-The following variables are available in templates:
-
-- `name` (canary.metadata.name)
-- `namespace` (canary.metadata.namespace)
-- `target` (canary.spec.targetRef.name)
-- `service` (canary.spec.service.name)
-- `ingress` (canary.spec.ingresRef.name)
-- `interval` (canary.spec.canaryAnalysis.metrics[].interval)
-
-A canary analysis metric can reference a template with `templateRef`:
+Reference the template in the canary analysis:
 
 ```yaml
  canaryAnalysis:
  metrics:
  - name: "404s percentage"
  templateRef:
  name: not-found-percentage
- # namespace is optional
- # when not specified, the canary namespace will be used
  namespace: istio-system
  thresholdRange:
  max: 5
@@ -134,4 +170,66 @@ spec:
  ) * 100
 ```
 
-The above template is for gPRC services instrumented with [go-grpc-prometheus](https://github.com/grpc-ecosystem/go-grpc-prometheus).
+The above template is for gPRC services instrumented with [go-grpc-prometheus](https://github.com/grpc-ecosystem/go-grpc-prometheus).
+
+### Datadog
+
+You can create custom metric checks using the Datadog provider.
+
+Create a secret with your Datadog API credentials:
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+ name: datadog
+ namespace: istio-system
+data:
+ datadog_api_key: your-datadog-api-key
+ datadog_application_key: your-datadog-application-key
+```
+
+Datadog template example:
+
+```yaml
+apiVersion: flagger.app/v1beta1
+kind: MetricTemplate
+metadata:
+ name: not-found-percentage
+ namespace: istio-system
+spec:
+ provider:
+ type: datadog
+ address: https://api.datadoghq.com
+ secretRef:
+ name: datadog
+ query: |
+ 100 - (
+ sum:istio.mesh.request.count{
+ reporter:destination,
+ destination_workload_namespace:{{ namespace }},
+ destination_workload:{{ target }},
+ !response_code:404
+ }.as_count()
+ / 
+ sum:istio.mesh.request.count{
+ reporter:destination,
+ destination_workload_namespace:{{ namespace }},
+ destination_workload:{{ target }}
+ }.as_count()
+ ) * 100
+```
+
+Reference the template in the canary analysis:
+
+```yaml
+ canaryAnalysis:
+ metrics:
+ - name: "404s percentage"
+ templateRef:
+ name: not-found-percentage
+ namespace: istio-system
+ thresholdRange:
+ max: 5
+ interval: 1m
+```