make x509_read_timestamp an optional metric

To preserve storage efficiency in TSDB systems that don't need the
metric for alerting.

Argument --timestamp-metric is to be used when x509_read_timestamp
should be emitted.

Signed-off-by: Thibault VINCENT <root@paraneko.org>

README.md

@@ -107,6 +107,7 @@ x509_cert_not_after - time()
 ```
 
 When collecting these metrics from tools like Datadog that does not have timestamp functions, the `x509_read_timestamp` allow to know read timestamp.
+Run the exporter with `--timestamp-metric` to get that optionnal metric.
 
 ```
 x509_cert_not_after - x509_read_timestamp > delay_in_seconds

cmd/x509-certificate-exporter/main.go

@@ -27,6 +27,7 @@ func main() {
 	port := getopt.IntLong("port", 'p', 9793, "prometheus exporter listening port")
 	debug := getopt.BoolLong("debug", 0, "enable debug mode")
 	trimPathComponents := getopt.IntLong("trim-path-components", 0, 0, "remove <n> leading component(s) from path(s) in label(s)")
+	emitTimestampMetric := getopt.BoolLong("timestamp-metric", 0, "provide an additionnal metric with current timestamp")
 
 	files := stringArrayFlag{}
 	getopt.FlagLong(&files, "watch-file", 'f', "watch one or more x509 certificate file")
@@ -77,6 +78,7 @@ func main() {
 		YAMLs:                 yamls,
 		YAMLPaths:             internal.DefaultYamlPaths,
 		TrimPathComponents:    *trimPathComponents,
+		EmitTimestampMetric:   *emitTimestampMetric,
 		KubeSecretTypes:       kubeSecretTypes,
 		KubeIncludeNamespaces: kubeIncludeNamespaces,
 		KubeExcludeNamespaces: kubeExcludeNamespaces,

internal/collector.go

@@ -72,11 +72,13 @@ func (collector *collector) Collect(ch chan<- prometheus.Metric) {
 		float64(len(certErrors)),
 	)
 
-	ch <- prometheus.MustNewConstMetric(
-		certTimestampDesc,
-		prometheus.GaugeValue,
-		float64(time.Now().Unix()),
-	)
+	if collector.exporter.EmitTimestampMetric {
+		ch <- prometheus.MustNewConstMetric(
+			certTimestampDesc,
+			prometheus.GaugeValue,
+			float64(time.Now().Unix()),
+		)
+	}
 }
 
 func (collector *collector) getMetricsForCertificate(certData *parsedCertificate, ref *certificateRef) []prometheus.Metric {

internal/exporter.go

@@ -22,6 +22,7 @@ type Exporter struct {
 	YAMLs                 []string
 	YAMLPaths             []YAMLCertRef
 	TrimPathComponents    int
+	EmitTimestampMetric   bool
 	KubeSecretTypes       []string
 	KubeIncludeNamespaces []string
 	KubeExcludeNamespaces []string