doc/user: Add monitoring with prometheus documentation

Author: lilic

doc/user/monitoring/monitoring-with-prometheus.md

@@ -0,0 +1,54 @@
+# Monitoring with Prometheus
+
+[Prometheus](https://prometheus.io/) is an open-source systems monitoring and alerting toolkit. Below is the overview of the different helpers that exist in operator-sdk to help setup metrics in operators.
+
+
+## operator-sdk specific metrics
+
+The `metrics.ExposeMetricsPort` is a helper function that creates and registers go and process collector metrics in operator-sdk. It also serves those metrics on `:6000/metrics`. A [Service](https://kubernetes.io/docs/concepts/services-networking/service/) object is created with the port exposed.
+
+### Usage:
+
+```go
+import "github.com/operator-framework/operator-sdk/pkg/metrics"
+
+// Returns the Service object and exposes metrics on port `6000/metrics`.
+s, err := metrics.ExposeMetricsPort()
+if err != nil {
+   log.Println(err)
+}
+client := mgr.GetClient()
+
+// Create the operator-sdk Service object.
+client.Create(context.TODO(), s)
+```
+
+### Usage:
+
+
+## Using ServiceMonitor prometheus-operator CRD
+
+[prometheus-operator](https://github.com/coreos/prometheus-operator) is an operator that creates, configures and manages Prometheus clusters atop Kubernetes.
+
+`ServiceMonitor` is a CRD of the prometheus-operator, which discovers the `Endpoints` in `Service` objects and configures Prometheus to monitor those `Pod`s. The `GenerateServiceMonitor` takes a `Service` object and generates a `ServiceMonitor` resource based on it.
+To add `Service` target discover of your created monitoring `Service`s you can use the `GenerateServiceMonitor` helper function, which accepts the newly created `Service`.
+
+### Prerequisites:
+
+- [prometheus-operator](https://github.com/coreos/prometheus-operator/tree/master/contrib/kube-prometheus#quickstart) needs to be deployed already in the cluster.
+
+### Usage example:
+
+```go
+import "github.com/operator-framework/operator-sdk/pkg/metrics"
+
+// Pass the Service to the helper function, which in turn returns the `ServiceMonitor` object.
+sm := metrics.GenerateServiceMonitor(service)
+
+// Create the `ServiceMonitor` resource in the cluster.
+client.Create(context.TODO(), s)
+sdk.Create(sm)
+```
+
+*Note:* Create one `ServiceMonitor` per application and per `Namespace`.
+