Add metrics API scaler docs

content/blog/keda-2.0-beta.md.draft

@@ -25,6 +25,7 @@ Here are some highlights:
     - Provide more information when quering KEDA resources with `kubectl`
 - **Extensibility**
     - Introduction of External Push scaler ([docs](https://keda.sh/docs/2.0/scalers/external-push/))
+    - Introduction of Metric API scaler ([docs](https://keda.sh/docs/2.0/scalers/metrics-api/))
     - Provide KEDA client-go library
 
 For a full list of changes, we highly recommend going through [our changelog](https://github.com/kedacore/keda/blob/v2/CHANGELOG.md#v200)! With our stable release, we'll provide a full overview of what's released in a new blog post.

content/docs/2.0/scalers/metrics-api.md

@@ -0,0 +1,80 @@
++++
+title = "Metrics API"
+layout = "scaler"
+availability = "v2.0+"
+maintainer = "Community"
+description = "Scale applications based on a metric provided by an API"
+go_file = "metrics_api_scaler"
++++
+
+### Trigger Specification
+
+This specification describes the `metrics-api` trigger that scales based on a metric value provided by an API. 
+
+This scaler allows users to utilize **any existing APIs** as a metric provider.  
+
+Here is an example of trigger configuration using metric-api scaler:
+
+```yaml
+triggers:
+- type: metric-api
+  metadata:
+    targetValue: "8"
+    url: "http://api:3232/api/v1/stats"
+    valueLocation: "components.worker.tasks"
+```
+
+**Parameter list:**
+- `url`: Full URL of the API operation to call to get the metric value (eg. `http://app:1317/api/v1/stats`).
+- `valueLocation`: [GJSON path notation](https://github.com/tidwall/gjson#path-syntax) to refer to the field in 
+    the payload containing the metric value
+- `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.
+
+### Authentication Parameters
+
+Not supported yet.
+
+### Example
+
+Here is a full example of scaled object definition using Metric API trigger:
+
+```yaml
+apiVersion: keda.sh/v1alpha1
+kind: ScaledObject
+metadata:
+  name: http-scaledobject
+  namespace: keda
+  labels:
+    deploymentName: dummy
+spec:
+  maxReplicaCount: 12
+  scaleTargetRef:
+    name: dummy
+  triggers:
+    - type: metrics-api
+      metadata:
+        targetValue: "7"
+        url: "http://api:3232/components/stats"
+        valueLocation: 'components.worker.tasks'
+```
+
+When checking current metric Metrics API scaler sends GET request to provided `url` and then uses `valueLocation`
+to access the value in response's payload. 
+
+The above example expects that the API endpoint will return response similar to this one:
+```json
+{
+  "components": {
+    "worker": {
+      "tasks": 12,
+      ...
+    },
+    ...
+  },
+  ...
+}
+```
+Assuming such response, Metrics API trigger will figure out that current metric value is 12.
+
+> NOTE: The value of the metric must be json number type. The value is casted to **integer**.