Add support for custom Prometheus metrics.

Start with a basic metric about the amount of managed machines.

pkg/controller/controller.go

@@ -30,6 +30,7 @@ import (
 	machineinformers "github.com/gardener/machine-controller-manager/pkg/client/informers/externalversions/machine/v1alpha1"
 	machinelisters "github.com/gardener/machine-controller-manager/pkg/client/listers/machine/v1alpha1"
 	"github.com/golang/glog"
+	"github.com/prometheus/client_golang/prometheus"
 	"k8s.io/api/core/v1"
 	coreinformers "k8s.io/client-go/informers/core/v1"
 	v1core "k8s.io/client-go/kubernetes/typed/core/v1"
@@ -411,6 +412,12 @@ func (c *controller) Run(workers int, stopCh <-chan struct{}) {
 	glog.V(1).Info("Starting machine-controller-manager")
 	handlers.UpdateHealth(true)
 
+	// The controller implement the prometheus.Collector interface and can therefore
+	// be passed to the metrics registry. Collectors which added to the registry
+	// will collect metrics to expose them via the metrics endpoint of the mcm
+	// every time when the endpoint is called.
+	prometheus.MustRegister(c)
+
 	for i := 0; i < workers; i++ {
 		createWorker(c.openStackMachineClassQueue, "ClusterOpenStackMachineClass", maxRetries, true, c.reconcileClusterOpenStackMachineClassKey, stopCh, &waitGroup)
 		createWorker(c.awsMachineClassQueue, "ClusterAWSMachineClass", maxRetries, true, c.reconcileClusterAWSMachineClassKey, stopCh, &waitGroup)

pkg/controller/metrics.go

@@ -0,0 +1,59 @@
+/*
+Copyright (c) 2018 SAP SE or an SAP affiliate company. All rights reserved.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package controller is used to provide the core functionalities of machine-controller-manager
+
+package controller
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	"k8s.io/apimachinery/pkg/labels"
+)
+
+var (
+	machineCountDesc = prometheus.NewDesc("mcm_machine_items_total", "Count of machines currently managed by the mcm.", nil, nil)
+
+	// ScrapeFailedCounter is a Prometheus metric, which counts errors during metrics collection.
+	ScrapeFailedCounter = prometheus.NewCounterVec(prometheus.CounterOpts{
+		Name: "mcm_scrape_failure_total",
+		Help: "Total count of scrape failures.",
+	}, []string{"kind"})
+)
+
+func init() {
+	prometheus.MustRegister(ScrapeFailedCounter)
+}
+
+// Describe is method required to implement the prometheus.Collect interface.
+func (c *controller) Describe(ch chan<- *prometheus.Desc) {
+	ch <- machineCountDesc
+}
+
+// Collect is method required to implement the prometheus.Collect interface.
+func (c *controller) Collect(ch chan<- prometheus.Metric) {
+	// Collect the count of machines managed by the mcm.
+	machineList, err := c.machineLister.Machines(c.namespace).List(labels.Everything())
+	if err != nil {
+		ScrapeFailedCounter.With(prometheus.Labels{"kind": "machine-count"}).Inc()
+		return
+	}
+	metric, err := prometheus.NewConstMetric(machineCountDesc, prometheus.GaugeValue, float64(len(machineList)))
+	if err != nil {
+		ScrapeFailedCounter.With(prometheus.Labels{"kind": "machine-count"}).Inc()
+		return
+	}
+	ch <- metric
+}