Add OpenStack metrics scaler doc

content/docs/2.3/scalers/openstack-metric.md

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+title = "OpenStack Metric"
+layout = "scaler"
+availability = "v2.3+"
+maintainer = "Community"
+description = "Scale applications based on a threshold reached by a specific measure from OpenStack Metric API."
+go_file = "openstack_metrics_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `openstack-metric` trigger for OpenStack metrics.
+> The OpenStack metric API follows [Gnocchi](https://gnocchi.xyz/) API. Attempt to the fact that Gnocchi API is an open-source time series database embedded in OpenStack system and every parameter on OpenStack Metric API follows its patterns but you don't need to reference anything from Gnocchi. It scales based on a specific measure from a given resource metric.
+It's highly recommended to check [Gnocchi](https://gnocchi.xyz/) docs.
+
+```yaml
+triggers:
+- type: openstack-metric
+  metadata:
+    metricsURL: http://localhost:8041/v1/metric #required
+    metricID: 003bb589-166d-439d-8c31-cbf098d863de #required
+    aggregationMethod: "mean" #required
+    granularity: 300 #required (seconds)
+    threshold: 1250 #required
+    timeout: 30 #optional
+```
+
+> Protocol (http or https) should always be provided when specifying URLs
+
+**Parameter list:**
+- `metricsURL` - The URL to check for the metrics API, based. It must contain the hostname, the metric port, the API version, and the resource ID. The pattern is: `http://<host>:<metric_port>/<openstack_metric_api_version>/<resource_id>/metric` 
+- `metricID` - The Id of the intendend metric
+- `aggregationMethod` - The aggregation method that will be used to calculate metrics, it must follows the configured possible metrics derived from gnocchi API like: `mean`, `min`, `max`, `std`, `sum`, `count`, the complete aggregation methods list can be found [here](https://gnocchi.xyz/rest.html#archive-policy).
+- `granularity` - The configured granularity from metric collection in seconds. it must follow the same value configured in OpenStack, but it must be coutned in seconds. Sample: If you have a 5 minutes time window granularity defined, so you must input a value of 300 seconds (5*60).
+- `threshold` - The target value that, when reached, will scale the application.
+- `timeout` - The timeout, in seconds, for the HTTP client requests that will query the Metric API. If not specified, the default value is `30` seconds. This must be an integer value. If `0` or negative value is provided it defaults to `300` milliseconds.
+
+### Authentication Parameters
+
+To authenticate, this scaler uses tokens. Tokens are automatically retrieved by the scaler from [Keystone](https://docs.openstack.org/keystone/latest/), the official OpenStack Identity Provider. You can provide your credentials using Secrets either by using the "password" method or the "application credentials" method. Both cases use `TriggerAuthentication`.
+
+#### Password
+
+- `authURL` - The Keystone authentication URL. The pattern is: `http://<host>:<keystone_port>/<keystone_version>/`
+- `userID` - The OpenStack project user ID
+- `password` - The password for the provided user
+- `projectID` - The OpenStack project ID
+
+#### Application Credentials
+
+- `authURL` - The Keystone authentication URL. The pattern is: `http://<host>:<keystone_port>/<keystone_version>/`
+- `appCredentialID` - The Application Credential ID
+- `appCredentialSecret` - The Application Credential secret
+
+### Example
+
+#### Password method
+
+Here is an example of how to deploy a scaled object with the `openstack-metric` scale trigger which uses `TriggerAuthentication` and the Password method from above.
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: openstack-secret-password
+  namespace: default
+type: Opaque
+data:
+  authURL: aHR0cDovL2xvY2FsaG9zdDo1MDAwL3YzLw==
+  userID: MWYwYzI3ODFiNDExNGQxM2E0NGI4ODk4Zjg1MzQwYmU=
+  password: YWRtaW4=
+  projectID: YjE2MWRjNTE4Y2QyNGJkYTg0ZDk0ZDlhMGU3M2ZjODc=
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: metrics-password-trigger-authentication
+  namespace: default
+spec:
+  secretTargetRef:
+  - parameter: authURL
+    name: openstack-secret-password
+    key: authURL
+  - parameter: userID
+    name: openstack-secret-password
+    key: userID
+  - parameter: password
+    name: openstack-secret-password
+    key: password
+  - parameter: projectID
+    name: openstack-secret-password
+    key: projectID
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: openstack-metric-scaledobject
+  namespace: default
+spec:
+  scaleTargetRef:
+    name: my-deployment
+  pollingInterval: 10
+  cooldownPeriod: 10
+  minReplicaCount: 0
+  triggers:
+  - type: openstack-metric
+    metadata:
+      metricsURL: http://localhost:8041/v1/metric 
+      metricID: faf01aa5-da88-4929-905d-b83fbab46771
+      aggregationMethod: "mean"
+      granularity: 300 
+      threshold: 1250 
+      timeout: 30 
+    authenticationRef:
+        name: openstack-metric-password-trigger-authentication
+```
+
+#### Application Credentials method
+
+You can also use the Application Credentials method. 
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: openstack-secret-appcredentials
+  namespace: default
+type: Opaque
+data:
+  authURL: aHR0cDovL2xvY2FsaG9zdDo1MDAwL3YzLw==
+  appCredentialID: OWYyY2UyYWRlYmFkNGQxNzg0NTgwZjE5ZTljMTExZTQ=
+  appCredentialSecret: LVdSbFJBZW9sMm91Z3VmZzNEVlBqcll6aU9za1pkZ3c4Y180XzRFU1pZREloT0RmajJkOHg0dU5yb3NudVIzWmxDVTZNLTVDT3R5NDFJX3M5R1N5Wnc=
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: openstack-metric-appcredentials-trigger-authentication
+  namespace: default
+spec:
+  secretTargetRef:
+  - parameter: authURL
+    name: openstack-secret-appcredentials
+    key: authURL
+  - parameter: appCredentialID
+    name: openstack-secret-appcredentials
+    key: appCredentialID
+  - parameter: appCredentialSecret
+    name: openstack-secret-appcredentials
+    key: appCredentialSecret
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: openstack-metric-scaledobject
+  namespace: default
+spec:
+  scaleTargetRef:
+    name: my-deployment
+  pollingInterval: 10
+  cooldownPeriod: 10
+  minReplicaCount: 0
+  triggers:
+  - type: openstack-metric
+    metadata:
+       metricsURL: http://localhost:8041/v1/metric 
+      metricID: faf01aa5-da88-4929-905d-b83fbab46771
+      aggregationMethod: "mean"
+      granularity: 300 
+      threshold: 1250
+    authenticationRef:
+        name: openstack-metric-appcredentials-trigger-authentication
+```