Doc: Tetragon metrics

Added how to read tetragon metrics with Kubernetes and Package
installation setup.

Signed-off-by: Prateek Singh <prateeksingh9741@gmail.com>

docs/content/en/docs/tutorials/tetragon-metrics.md

@@ -0,0 +1,104 @@
+---
+title: "Tetragon Metrics"
+weight: 1
+icon: "overview"
+description: "Fetching and understanding Tetragon metrics"
+---
+
+## Kubernetes
+
+When installed with Helm as described in 
+[Deploying on Kubernetes]({{< ref 
+"/docs/getting-started/deployment/kubernetes" >}}), Tetragon pods expose a 
+metrics endpoint by default. The chart also creates a service named `tetragon`
+that exposes metrics on the specified port. 
+
+### Getting metrics port
+
+Check if the `tetragon` service exists:
+
+```shell-session
+kubectl get services tetragon -n kube-system
+```
+
+The output should be similar to:
+```
+NAME       TYPE        CLUSTER-IP     EXTERNAL-IP   PORT(S)     AGE
+tetragon   ClusterIP   10.96.54.218   <none>        2112/TCP    3m
+```
+
+{{< note >}}
+In the previous output it shows, 2112 is the port on which the service is 
+listening. It is also the port on which the Tetragon metrics server listens 
+with the default Helm values.
+{{< /note >}}
+
+### Port Forwarding
+
+To forward the metrics port locally, use `kubectl port forward`:
+
+```shell-session
+kubectl -n kube-system port-forward service/tetragon 2112:2112
+```
+
+## Package
+
+Tetragon, when installed via release packages as mentioned in 
+[Package Deployment]({{< ref "/docs/getting-started/deployment/package" >}}). 
+By default, metrics are disabled, which can be enabled using `--metrics-server` 
+flag, by specifying the address.
+
+Alternatively, the [examples/configuration/tetragon.yaml](https://github.com/cilium/tetragon/blob/main/examples/configuration/tetragon.yaml)
+file contains example entries showing the defaults for the address of 
+metrics-server. Local overrides can be created by editing and copying this file
+into `/etc/tetragon/tetragon.yaml`, or by editing and copying "drop-ins" from
+the [examples/configuration/tetragon.conf.d](https://github.com/cilium/tetragon/tree/main/examples/configuration/tetragon.conf.d)
+directory into the `/etc/tetragon/tetragon.conf.d/` subdirectory. The latter is
+generally recommended.
+
+### Set Metrics Address
+
+Run `sudo tetragon --metrics-server localhost:2112` to set metrics address to `localhost:2112` and export metrics.
+
+```shell-session
+sudo tetragon --metrics-server localhost:2112
+```
+
+The output should be similar to this:
+
+```
+time="2023-09-21T13:17:08+05:30" level=info msg="Starting tetragon" 
+version=v0.11.0
+time="2023-09-21T13:17:08+05:30" level=info msg="config settings" 
+config="mapeased
+time="2023-09-22T23:16:24+05:30" level=info msg="Starting metrics server" 
+addr="localhost:2112" 
+[...]
+time="2023-09-21T13:17:08+05:30" level=info msg="Listening for events..."
+```
+
+Alternatively, a file named `server-address` can be created in `etc/tetragon/tetragon.conf.d/metrics-server` with content specifying 
+a port like this `localhost:2112`, or any port of your choice as mentioned 
+above.
+
+## Fetch the Metrics
+
+After the metrics are exposed, either by port forwarding in case of 
+Kubernetes installation or by setting metrics address in case of Package 
+installation, the metrics can be fetched using 
+`curl` on `localhost:2112/metrics`:
+
+```shell-session
+curl localhost:2112/metrics
+```
+
+The output should be similar to this:
+```
+# HELP promhttp_metric_handler_errors_total Total number of internal errors encountered by the promhttp metric handler.
+# TYPE promhttp_metric_handler_errors_total counter
+promhttp_metric_handler_errors_total{cause="encoding"} 0
+promhttp_metric_handler_errors_total{cause="gathering"} 0
+# HELP tetragon_errors_total The total number of Tetragon errors. For internal use only.
+# TYPE tetragon_errors_total counter
+[...]
+```