doc/user/metrics: Add metrics docs

Author: lilic

doc/user/metrics/README.md

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+# 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.
+
+## Metrics in operator-sdk
+
+The `metrics.ExposeMetricsPort(metricsAddress)` is a helper function that exposes general metrics about the running program. The metrics are by default served on `:8383/metrics`. A [Service](https://kubernetes.io/docs/concepts/services-networking/service/) object is created with the metrics port exposed.
+
+To modify the port the metrics are exposed on, change the `var metricsAddress string = ":8383"` variable in the `cmd/manager/main.go` file of the generated operator.
+
+### Usage:
+
+```go
+    import "github.com/operator-framework/operator-sdk/pkg/metrics"
+
+    // Change metricsAddress to serve metrics on different host and port.
+    var metricsAddress string = ":8383"
+
+    // Pass metricsAddress when initiliazing the manager.
+    mgr, err := manager.New(cfg, manager.Options{Namespace: namespace, MetricsBindAddress: metricsAddress})
+
+	// Expose metrics by creating a Service object
+	s, err := metrics.ExposeMetricsPort(metricsAddress)
+	if err != nil {
+        // handle error
+	}
+	client := mgr.GetClient()
+	if err := client.Create(context.TODO(), s); err != nil && !apierrors.IsAlreadyExists(err) {
+	    // handle error
+    }
+
+```
+
+Note: The above example is already present in `cmd/manager/main.go` in all the operators generated with operator-sdk.

doc/user/metrics/service_monitor.md

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+## Using the 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 [CR](https://kubernetes.io/docs/concepts/extend-kubernetes/api-extension/custom-resources/) 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 discovery of your created monitoring `Service`s you can use the `metrics.CreateServiceMonitor()` 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(s) to the helper function, which in turn returns the `ServiceMonitor` object.
+serviceMonitors, err := metrics.CreateServiceMonitors(restConfig, "namespace", services)
+if err != nil {
+    // handle error here
+}
+```
+
+*Note:* Create one `ServiceMonitor` per application and per `Namespace`.
+