Add metrics for config file changes

This uses code pieces from prometheus/alertmanager in https://github.com/prometheus/alertmanager/blob/main/config/coordinator.go#LL56C26-L56C26
licensed under Apache-2.0.

kube_state_metrics_config_hash 4.0061079457904e+13
kube_state_metrics_config_last_reload_success_timestamp_seconds 1.6697483049487052e+09
kube_state_metrics_config_last_reload_successful 1

Signed-off-by: Manuel Rüger <manuel@rueg.eu>

pkg/app/server.go

@@ -18,6 +18,8 @@ package app
 
 import (
 	"context"
+	"crypto/md5" //nolint:gosec
+	"encoding/binary"
 	"fmt"
 	"net"
 	"net/http"
@@ -99,6 +101,22 @@ func RunKubeStateMetrics(ctx context.Context, opts *options.Options, factories .
 			ConstLabels: prometheus.Labels{"handler": "metrics"},
 		}, []string{"method"},
 	)
+	configHash := promauto.With(ksmMetricsRegistry).NewGauge(
+		prometheus.GaugeOpts{
+			Name: "kube_state_metrics_config_hash",
+			Help: "Hash of the currently loaded configuration.",
+		})
+	configSuccess := promauto.With(ksmMetricsRegistry).NewGauge(
+		prometheus.GaugeOpts{
+			Name: "kube_state_metrics_config_last_reload_successful",
+			Help: "Whether the last configuration reload attempt was successful.",
+		})
+	configSuccessTime := promauto.With(ksmMetricsRegistry).NewGauge(
+		prometheus.GaugeOpts{
+			Name: "kube_state_metrics_config_last_reload_success_timestamp_seconds",
+			Help: "Timestamp of the last successful configuration reload.",
+		})
+
 	storeBuilder.WithMetrics(ksmMetricsRegistry)
 
 	got := options.GetOptsConfigFile(*opts)
@@ -117,6 +135,12 @@ func RunKubeStateMetrics(ctx context.Context, opts *options.Options, factories .
 			klog.Infof("misconfigured config detected, KSM will automatically reload on next write to the config")
 			klog.Infof("waiting for config to be fixed")
 			<-ctx.Done()
+			configSuccess.Set(0)
+		} else {
+			configSuccess.Set(1)
+			configSuccessTime.SetToCurrentTime()
+			hash := md5HashAsMetricValue(optsConfigFile)
+			configHash.Set(hash)
 		}
 	}
 	var resources []string
@@ -371,3 +395,12 @@ func buildMetricsServer(m *metricshandler.MetricsHandler, durationObserver prome
 	})
 	return mux
 }
+
+func md5HashAsMetricValue(data []byte) float64 {
+	sum := md5.Sum(data) //nolint:gosec
+	// We only want 48 bits as a float64 only has a 53 bit mantissa.
+	smallSum := sum[0:6]
+	bytes := make([]byte, 8)
+	copy(bytes, smallSum)
+	return float64(binary.LittleEndian.Uint64(bytes))
+}

pkg/options/options.go

@@ -62,7 +62,7 @@ type Options struct {
 	cmd *cobra.Command
 }
 
-// GetOptsConfigFile is the getter for --options-config-file value.
+// GetOptsConfigFile is the getter for --config value.
 func GetOptsConfigFile(opt Options) string {
 	return opt.Config
 }