instrument k8s-prometheus-adapter to expose prometheus metrics

fixes #218

pkg/metrics/metrics.go

@@ -0,0 +1,38 @@
+package metrics
+
+import "github.com/prometheus/client_golang/prometheus"
+
+const MetricsNamespace = "adapter"
+
+var PrometheusUp = prometheus.NewGauge(prometheus.GaugeOpts{
+	Namespace: MetricsNamespace,
+	Name:      "prometheus_up",
+	Help:      "1 when adapter is able to reach prometheus, 0 otherwise",
+})
+
+var RegistryMetrics = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+	Namespace: MetricsNamespace,
+	Name:      "registry_metrics",
+	Help:      "number of metrics entries in cache registry",
+}, []string{"registry"})
+
+var Errors = prometheus.NewCounterVec(prometheus.CounterOpts{
+	Namespace: MetricsNamespace,
+	Name:      "errors_total",
+	Help:      "number of errors served",
+}, []string{"type"})
+
+var Rules = prometheus.NewGaugeVec(prometheus.GaugeOpts{
+	Namespace: MetricsNamespace,
+	Name:      "roles",
+	Help:      "number of configured rules",
+}, []string{"type"})
+
+func init() {
+	prometheus.MustRegister(
+		PrometheusUp,
+		RegistryMetrics,
+		Errors,
+		Rules,
+	)
+}