Document the sidecar.istio.io/statsInclusionPrefixes annotation (#3581)

* Document the sidecar.istio.io/statsInclusionPrefixes annotation

* Minor wording changes suggested by venilnoronha

* Remove trailing spaces to satisfy lint

* Use code style for filenames

* Address two concerns from Frank

* Incorporate comments from rcaballeromx

* Remove trailing space at end of line

content/help/ops/telemetry/envoy-stats/index.md

@@ -0,0 +1,47 @@
+---
+title: Envoy Statistics
+description: Fine-grained control of Envoy statistics.
+weight: 95
+---
+
+The Envoy proxy keeps detailed statistics about network traffic.
+
+Envoy's statistics only cover the traffic for a particular Envoy instance.  See
+[Telemetry](/docs/tasks/telemetry/) for persistent per-service Istio telemetry.  The
+statistics the Envoy proxies record can provide more information about specific pod instances.
+
+To see the statistics for a pod:
+
+{{< text bash >}}
+$ kubectl exec -it $POD  -c istio-proxy  -- sh -c 'curl localhost:15000/stats'
+{{< /text >}}
+
+See [the Envoy documentation](https://www.envoyproxy.io/docs/envoy/latest/configuration/cluster_manager/cluster_stats)
+for an explanation of the data recorded.
+
+By default, Istio configures Envoy to record minimal statistics.  The default collection
+keys are:
+
+- `cluster_manager`
+- `listener_manager`
+- `http_mixer_filter`
+- `tcp_mixer_filter`
+- `server`
+- `cluster.xds-grpc`
+
+To see the Envoy settings for statistics data collection use
+`istioctl proxy-config bootstrap` and follow the
+[deep dive into Envoy configuration](/help/ops/traffic-management/proxy-cmd/#deep-dive-into-envoy-configuration).
+Envoy only collects statistical data on items matching the `inclusion_list` within
+the `stats_matcher` JSON element.
+
+To Configure Envoy to record statistics for inbound or outbound traffic, add the
+`sidecar.istio.io/statsInclusionPrefixes` annotation to the pod template in the Kubernetes `Deployment`.
+Add the `cluster.outbound` prefix to gather data about outbound traffic activity and circuit breaking.
+To gather data on inbound traffic, add the `listener` prefix.  The sample
+[fortio-deploy.yaml]({{< github_file>}}/samples/httpbin/sample-client/fortio-deploy.yaml)
+shows use of `sidecar.istio.io/statsInclusionPrefixes` with the `cluster.outbound` prefix.
+
+You can override the Envoy defaults to gather less data than usual.  Use
+`sidecar.istio.io/statsInclusionPrefixes: cluster_manager,listener_manager`
+to collect the least statistics possible.

content/help/ops/telemetry/missing-metrics/index.md

@@ -1,6 +1,6 @@
 ---
 title: Missing Metrics
-description:
+description: Diagnose problems where metrics are not being collected.
 weight: 10
 ---