Docs -> Steps to Deploy Test App (#2788)

* Docs-> deploy test app

* fix review comments

* Add intro to go_app README

---------

Co-authored-by: Silvestre Zabala <[email protected]>

src/acceptance/assets/app/go_app/README.md

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+# ASATS test app
+
+This app is used in the app-autoscaler acceptance test suite (ASATS) to generate metrics to test end-to-end scenarios.
+
+The app can be made to consume and release on demand:
+- main memory 
+- CPU
+- hard disk
+
+It can be used to emit custom metrics.
+
+It can be used to respond to HTTP requests:
+- as fast as possible
+- or with a configurable request latency
+
+**The [API](https://github.com/cloudfoundry/app-autoscaler-release/blob/main/src/acceptance/assets/app/go_app/internal/app/app.go#L49-L54) of the test app is not part of the public API and may change at any time.**
+
+Still, the app can be useful during development and one such use case is described in detail in the next section.
+
+## Deploy Test Application with Custom Metrics
+
+### Sample Autoscaling Policy
+
+Autoscaling policy is available at [test-policy.json](example/test-policy.json)
+
+```json
+{
+  "instance_min_count": 1,
+  "instance_max_count": 2,
+  "scaling_rules": [
+    {
+      "metric_type": "review_count",
+      "breach_duration_secs": 60,
+      "threshold": 200,
+      "operator": ">=",
+      "cool_down_secs": 60,
+      "adjustment": "+1"
+    },
+    {
+      "metric_type": "review_count",
+      "breach_duration_secs": 60,
+      "threshold": 100,
+      "operator": "<=",
+      "cool_down_secs": 60,
+      "adjustment": "-1"
+    }
+  ]
+}
+```
+
+### Deploy Test Application
+
+Prepare acceptance_config.json. Example [acceptance_config.json](../../../example_config/acceptance_config.json)
+
+```bash
+CONFIG=$PWD/acceptance_config.json make deploy-test-app && \
+#attach autoscaling policy (as defined above)
+cf attach-autoscaling-policy test_app test-policy.json && \
+# show autoscaling policy using autoscaler cli plugin
+cf asp test_app
+```
+
+### Send Custom Metrics
+
+```bash
+# scale out 
+APP_ROUTE=$(cf app test_app | grep routes | awk '{print $2}')
+curl https://$APP_ROUTE/custom-metrics/mtls/review_count/201
+
+# scale In 
+APP_ROUTE=$(cf app test_app | grep routes | awk '{print $2}')
+curl https://$APP_ROUTE/custom-metrics/mtls/review_count/78
+```
+
+
+### View Metrics in Log Cache
+
+Metrics are ingested into log-cache and can be viewed using [CF Log Cache CLI Plugin](https://github.com/cloudfoundry/log-cache-cli)
+
+```bash
+cf tail --name-filter="review_count" test_app --follow
+```

src/acceptance/assets/app/go_app/example/test-policy.json

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+{
+  "instance_min_count": 1,
+  "instance_max_count": 2,
+  "scaling_rules": [
+    {
+      "metric_type": "review_count",
+      "breach_duration_secs": 60,
+      "threshold": 200,
+      "operator": ">=",
+      "cool_down_secs": 60,
+      "adjustment": "+1"
+    },
+    {
+      "metric_type": "review_count",
+      "breach_duration_secs": 60,
+      "threshold": 100,
+      "operator": "<=",
+      "cool_down_secs": 60,
+      "adjustment": "-1"
+    }
+  ]
+}