Added documentation for the new GCP stackdriver scaler (kedacore#713)

Signed-off-by: Ram Cohen <ram.cohen@gmail.com>

content/docs/2.7/scalers/gcp-stackdriver.md

@@ -0,0 +1,117 @@
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+title = "Google Cloud Platform Stackdriver"
+availability = "2.7+"
+maintainer = "Community"
+description = "Scale applications based on a metric obtained from Stackdriver."
+layout = "scaler"
+go_file = "gcp_stackdriver_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `gcp-stackdriver` trigger for GCP Stackdriver. It scales based on a metric obtained from issuing a query to Stackdriver.
+
+```yaml
+triggers:
+- type: gcp-stackdriver
+  metadata:
+    projectId: my-project-id
+    filter: 'metric.type="storage.googleapis.com/network/received_bytes_count" AND resource.type="gcs_bucket" AND metric.label.method="WriteObject" AND resource.label.bucket_name="my-gcp-bucket"'
+    targetValue: '100'
+    credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON
+```
+
+**Parameter list:**
+
+- `projectId` - GCP project Id that contains the metric.
+- `filter` - The stackdriver query filter for obtaining the metric. The metric is for the last minute and if multiple values are returned, the first one is used.
+- `targetValue` - Average target value to trigger scaling actions. (Default: `5`, Optional)
+
+The `credentialsFromEnv` property maps to the name of an environment variable in the scale target (`scaleTargetRef`) that contains the service account credentials (JSON). KEDA will use those to connect to Google Cloud Platform and collect the configured stack driver metrics.
+
+### Authentication Parameters
+You can use `TriggerAuthentication` CRD to configure the authenticate by providing the service account credentials in JSON. 
+
+**Credential based authentication:**
+
+- `GoogleApplicationCredentials` - Service account credentials in JSON.
+
+**Identity based authentication:**
+
+You can also use `TriggerAuthentication` CRD to configure the authentication using the associated service account of the running machine in Google Cloud. You only need to create a `TriggerAuthentication` as this example, and reference it in the `ScaledObject`. `ClusterTriggerAuthentication` can also be used if you intend to use it globally in your cluster.
+
+### Examples
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: gcp-stackdriver-scaledobject
+  namespace: keda-gcp-stackdriver-test
+spec:
+  scaleTargetRef:
+    name: keda-gcp-stackdriver-go
+  triggers:
+  - type: gcp-stackdriver
+    metadata:
+      projectId: my-project-id
+      filter: 'metric.type="storage.googleapis.com/network/received_bytes_count" AND resource.type="gcs_bucket" AND metric.label.method="WriteObject" AND resource.label.bucket_name="my-gcp-bucket"'
+      targetValue: "5"
+      credentialsFromEnv: GOOGLE_APPLICATION_CREDENTIALS_JSON
+```
+
+#### Use TriggerAuthentication with Kubernetes secret
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-gcp-credentials
+spec:
+  secretTargetRef:
+  - parameter: GoogleApplicationCredentials 
+    name: gcp-stackdriver-secret        # Required. Refers to the name of the secret
+    key: GOOGLE_APPLICATION_CREDENTIALS_JSON       # Required.
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: gcp-stackdriver-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-gcp-stackdriver-go
+  triggers:
+  - type: gcp-stackdriver
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      projectId: my-project-id
+      filter: 'metric.type="storage.googleapis.com/network/received_bytes_count" AND resource.type="gcs_bucket" AND metric.label.method="WriteObject" AND resource.label.bucket_name="my-gcp-bucket"'
+```
+
+#### Use TriggerAuthentication with GCP Identity
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-gcp-credentials
+spec:
+  podIdentity:
+    provider: gcp
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: gcp-stackdriver-scaledobject
+spec:
+  scaleTargetRef:
+    name: keda-gcp-stackdriver-go
+  triggers:
+  - type: gcp-stackdriver
+    authenticationRef:
+      name: keda-trigger-auth-gcp-credentials
+    metadata:
+      projectId: my-project-id
+      filter: 'metric.type="storage.googleapis.com/network/received_bytes_count" AND resource.type="gcs_bucket" AND metric.label.method="WriteObject" AND resource.label.bucket_name="my-gcp-bucket"'
+```