continue running when querying the jenkins api fails

Instead of terminating when the Jenkins Server can not be reachted, the jenkins
exporter continues to run.
This prevents that the jenkins-exporter terminates when e.g. the jenkins-server
is restarted.

A new Prometheus metric jenkins_exporter_errors is added, that counts the number
of encountered errors when trying to query the jenkins api.

README.md

@@ -17,9 +17,14 @@ It provides the following Prometheus metrics:
       - building_duration
       - executing_time
       - waiting_time
-
-By default metrics are recorded for every finished Jenkins build.
-The jobs for that builds are recorded can be limited with the
+- Counter Metric: `jenkins_exporter_errors`  
+  Counts the number of errors the jenkins-exporter encountered when fetching
+  informations from the Jenkins API.
+  - type:
+    - jenkins_api
+
+By default metrics are recorded for every finished Jenkins build. The Jenkins
+jobs for that builds are recorded can be limited with the
 `--jenkins-job-whitelist` command-line parameter.
 The duration types that are recorded can also be configured via a commandline
 parameter.

internal/prometheus/prometheus.go

@@ -12,6 +12,7 @@ type Collector struct {
 	namespace   string
 	summaries   map[string]*prometheus.SummaryVec
 	histograms  map[string]*prometheus.HistogramVec
+	counters    map[string]*prometheus.CounterVec
 	constLabels prometheus.Labels
 }
 
@@ -20,10 +21,32 @@ func NewCollector(namespace string, constLabels map[string]string) *Collector {
 		namespace:   namespace,
 		summaries:   map[string]*prometheus.SummaryVec{},
 		histograms:  map[string]*prometheus.HistogramVec{},
+		counters:    map[string]*prometheus.CounterVec{},
 		constLabels: constLabels,
 	}
 }
 
+func (c *Collector) CounterAdd(key string, cnt float64, help string, labels map[string]string) {
+	s, exist := c.counters[key]
+	if !exist {
+		s = prometheus.NewCounterVec(
+			prometheus.CounterOpts{
+				Namespace:   c.namespace,
+				Name:        sanitize(key),
+				ConstLabels: c.constLabels,
+				Help:        help,
+			},
+			labelNames(labels),
+		)
+
+		prometheus.MustRegister(s)
+
+		c.counters[key] = s
+	}
+
+	s.With(labels).Add(cnt)
+}
+
 func (c *Collector) Summary(key string, val float64, help string, labels map[string]string) {
 	s, exist := c.summaries[key]
 	if !exist {

main.go

@@ -24,7 +24,7 @@ import (
 
 const (
 	appName = "jenkins-exporter"
-	version = "0.1"
+	version = "0.2"
 )
 
 const (
@@ -297,6 +297,7 @@ func validateFlags() {
 func main() {
 	envy.Parse("JE")
 	flag.Parse()
+
 	if *printVersion {
 		fmt.Printf("%s\n", version)
 		os.Exit(0)
@@ -332,13 +333,14 @@ func main() {
 		WithLogger(debugLogger).
 		WithTimeout(timeout)
 
+	// create the counter with a 0  counter
+	collector.CounterAdd("errors", 0, "jenkins api fetch errors", map[string]string{"type": "jenkins_api"})
+
 	for {
 		err := fetchAndRecord(clt, stateStore, collector)
 		if err != nil {
 			log.Printf("fetching and recording builds metrics failed: %s", err)
-			if _, ok := err.(*jenkins.ErrHTTPRequestFailed); !ok {
-				os.Exit(1)
-			}
+			collector.CounterAdd("errors", 1, "jenkins api fetch errors", map[string]string{"type": "jenkins_api"})
 		}
 
 		logger.Printf("fetching and recording the next build metrics in %s", pollInterval)