feat: Introduce activationThreshold/minMetricValue for Metrics API Sc…

…aler (kedacore#823)

content/docs/2.8/scalers/metrics-api.md

@@ -20,6 +20,7 @@ triggers:
 - type: metrics-api
   metadata:
     targetValue: "8.8"
+    activationTargetValue: "3.8"
     url: "http://api:3232/api/v1/stats"
     valueLocation: "components.worker.tasks"
 ```
@@ -29,6 +30,7 @@ triggers:
 - `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. (This value can be a float)
+- `activationTargetValue` - Target value for activating the scaler. Learn more about activation [here](./../concepts/scaling-deployments.md#activating-and-scaling-thresholds).(Default: `0`, Optional, This value can be a float)
 
 ### Authentication Parameters