Add documentation for Elasticsearch scaler (#586)

Signed-off-by: Nicolas L <[email protected]>

content/docs/2.5/scalers/elasticsearch.md

@@ -0,0 +1,93 @@
++++
+title = "Elasticsearch"
+availability = "v2.5+"
+maintainer = "Community"
+description = "Scale applications based on elasticsearch search template query result."
+layout = "scaler"
+go_file = "elasticsearch_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `elasticsearch` trigger that scales based on result of an [elasticsearch search template](https://www.elastic.co/guide/en/elasticsearch/reference/current/search-template.html) query.
+
+The trigger always requires the following information:
+
+```yaml
+triggers:
+  - type: elasticsearch
+    metadata:
+      addresses: "http://localhost:9200"
+      username: "elastic"
+      passwordFromEnv: "ELASTIC_PASSWORD"
+      index: "my-index"
+      searchTemplateName: "my-search-template-name"
+      params: "param1:value1;param2:value2"
+      valueLocation: "hits.total.value"
+      targetValue: "1"
+```
+
+**Parameter list:**
+
+- `addresses` - Comma separated list of hosts and ports of the Elasticsearch cluster client nodes.
+- `username` - Username to authenticate with to Elasticsearch cluster.
+- `passwordFromEnv` - Environment variable to read the authentication password from to authenticate with the Elasticsearch cluster.
+- `index` - Comma separated list of indexes to run the search template query on.
+- `searchTemplateName` - The search template name to run.
+- `targetValue` - Target value to scale on. When the metric provided by the API is equal or higher to this value, KEDA will start scaling out. When the metric is 0 or less, KEDA will scale down to 0.
+- `parameters` - Parameters that will be used by the search template. It supports multiples params separated by a semicolon character ( `;` ).
+- `valueLocation` - [GJSON path notation](https://github.com/tidwall/gjson#path-syntax) to refer to the field in the payload containing the metric value.
+- `unsafeSsl` - Skip certificate validation when connecting over HTTPS. (Values: `true`, `false`, Default: `false`, Optional)
+
+### Authentication Parameters
+
+You can authenticate by using a username/password authentication.
+
+**Password Authentication:**
+
+- `username` - Username to authenticate with to Elasticsearch cluster.
+- `password` - Password for configured user to login to Elasticsearch cluster.
+
+### Example
+
+Here is an example of how to deploy a scaled object with the `elasticsearch` scale trigger which uses `TriggerAuthentication`.
+
+```yaml
+apiVersion: v1
+kind: Secret
+metadata:
+  name: elasticsearch-secrets
+type: Opaque
+data:
+  password: cGFzc3cwcmQh
+---
+apiVersion: keda.sh/v1alpha1
+kind: TriggerAuthentication
+metadata:
+  name: keda-trigger-auth-elasticsearch-secret
+spec:
+  secretTargetRef:
+  - parameter: password
+    name: elasticsearch-secrets
+    key: password
+---
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: elasticsearch-scaledobject
+spec:
+  scaleTargetRef:
+    name: "deployment-name"
+  triggers:
+    - type: elasticsearch
+      metadata:
+        addresses: "http://localhost:9200"
+        username: "elastic"
+        index: "my-index"
+        searchTemplateName: "my-search-template"
+        valueLocation: "hits.total.value"
+        targetValue: "10"
+        params: "dummy_value:1"
+      authenticationRef:
+        name: keda-trigger-auth-elasticsearch-secret
+```