Adding pubsub auth trigger

Signed-off-by: Harsh Thakur <[email protected]>

content/docs/2.0/scalers/gcp-pub-sub.md

@@ -26,11 +26,17 @@ The `credentialsFromEnv` property maps to the name of an environment variable in
 
 `subscriptionName` defines the subscription that should be monitored. The `subscriptionSize` determines the target average which the deployment will be scaled on. The default `subscriptionSize` is 5.
 
+Here's an [example](https://github.com/kedacore/sample-go-gcppubsub). 
+
 ### Authentication Parameters
+You can use `TriggerAuthentication` CRD to configure the authenticate by providing the service account credentials in JSON. 
+
+
+**Credential based authentication:**
 
-Not supported yet.
+- `GoogleApplicationCredentials` - Service account credentials in JSON
 
-### Example
+### Example 
 
 ```yaml
 apiVersion: keda.sh/v1alpha1
@@ -48,3 +54,31 @@ spec:
       subscriptionName: "mysubscription" # Required
       credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON # Required
 ```
+
+### Example using TriggerAuthentication
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-gcp-credentials
+spec:
+  secretTargetRef:
+  - parameter: GoogleApplicationCredentials 
+    name: pubsub-secret        # Required. Refers to the name of the secret
+    key: GOOGLE_APPLICATION_CREDENTIALS_JSON       # Required.
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: pubsub-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-pubsub-go
+  triggers:
+  - type: gcp-pubsub
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      subscriptionName: "input" # Required  
+```