Add up metrics

Add nginx_up and nginxplus_up metrics that show the status of the
last metric scrape from NGINX or NGINX Plus.

README.md

@@ -77,7 +77,11 @@ Usage of ./nginx-prometheus-exporter:
 
 ### Exported Metrics
 
-* For NGINX, all stub_status metrics are exported. Connect to the `/metrics` page of the running exporter to see the complete list of metrics along with their descriptions.
+* For NGINX, the following metrics are exported:
+    * All [stub_status](http://nginx.org/en/docs/http/ngx_http_stub_status_module.html) metrics.
+    * `nginx_up` -- shows the status of the last metric scrape: `1` for a successful scrape and `0` for a failed one.
+
+    Connect to the `/metrics` page of the running exporter to see the complete list of metrics along with their descriptions.
 * For NGINX Plus, the following metrics are exported:
     * [Connections](http://nginx.org/en/docs/http/ngx_http_api_module.html#def_nginx_connections).
     * [HTTP](http://nginx.org/en/docs/http/ngx_http_api_module.html#http_).
@@ -86,6 +90,7 @@ Usage of ./nginx-prometheus-exporter:
     * [Stream Server Zones](http://nginx.org/en/docs/http/ngx_http_api_module.html#def_nginx_stream_server_zone).
     * [HTTP Upstreams](http://nginx.org/en/docs/http/ngx_http_api_module.html#def_nginx_http_upstream). Note: for the `state` metric, the string values are converted to float64 using the following rule: `"up"` -> `1.0`, `"draining"` -> `2.0`, `"down"` -> `3.0`, `"unavail"` –> `4.0`, `"checking"` –> `5.0`, `"unhealthy"` -> `6.0`.
     * [Stream Upstreams](http://nginx.org/en/docs/http/ngx_http_api_module.html#def_nginx_stream_upstream). Note: for the `state` metric, the string values are converted to float64 using the following rule: `"up"` -> `1.0`, `"down"` -> `3.0`, `"unavail"` –> `4.0`, `"checking"` –> `5.0`, `"unhealthy"` -> `6.0`.
+    * `nginxplus_up` -- shows the status of the last metric scrape: `1` for a successful scrape and `0` for a failed one.
 
 
     Connect to the `/metrics` page of the running exporter to see the complete list of metrics along with their descriptions. Note: to see server zones related metrics you must configure [status zones](https://nginx.org/en/docs/http/ngx_http_status_module.html#status_zone) and to see upstream related metrics you must configure upstreams with a [shared memory zone](http://nginx.org/en/docs/http/ngx_http_upstream_module.html#zone).

collector/helper.go

@@ -2,6 +2,17 @@ package collector
 
 import "github.com/prometheus/client_golang/prometheus"
 
+const nginxUp = 1
+const nginxDown = 0
+
 func newGlobalMetric(namespace string, metricName string, docString string) *prometheus.Desc {
 	return prometheus.NewDesc(namespace+"_"+metricName, docString, nil, nil)
 }
+
+func newUpMetric(namespace string) prometheus.Gauge {
+	return prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: namespace,
+		Name:      "up",
+		Help:      "Status of the last metric scrape",
+	})
+}

collector/nginx.go

@@ -12,6 +12,7 @@ import (
 type NginxCollector struct {
 	nginxClient *client.NginxClient
 	metrics     map[string]*prometheus.Desc
+	upMetric    prometheus.Gauge
 	mutex       sync.Mutex
 }
 
@@ -28,12 +29,15 @@ func NewNginxCollector(nginxClient *client.NginxClient, namespace string) *Nginx
 			"connections_waiting":  newGlobalMetric(namespace, "connections_waiting", "Idle client connections"),
 			"http_requests_total":  newGlobalMetric(namespace, "http_requests_total", "Total http requests"),
 		},
+		upMetric: newUpMetric(namespace),
 	}
 }
 
 // Describe sends the super-set of all possible descriptors of NGINX metrics
 // to the provided channel.
 func (c *NginxCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.upMetric.Desc()
+
 	for _, m := range c.metrics {
 		ch <- m
 	}
@@ -46,10 +50,15 @@ func (c *NginxCollector) Collect(ch chan<- prometheus.Metric) {
 
 	stats, err := c.nginxClient.GetStubStats()
 	if err != nil {
+		c.upMetric.Set(nginxDown)
+		ch <- c.upMetric
 		log.Printf("Error getting stats: %v", err)
 		return
 	}
 
+	c.upMetric.Set(nginxUp)
+	ch <- c.upMetric
+
 	ch <- prometheus.MustNewConstMetric(c.metrics["connections_active"],
 		prometheus.GaugeValue, float64(stats.Connections.Active))
 	ch <- prometheus.MustNewConstMetric(c.metrics["connections_accepted"],

collector/nginx_plus.go

@@ -18,6 +18,7 @@ type NginxPlusCollector struct {
 	streamServerZoneMetrics     map[string]*prometheus.Desc
 	streamUpstreamMetrics       map[string]*prometheus.Desc
 	streamUpstreamServerMetrics map[string]*prometheus.Desc
+	upMetric                    prometheus.Gauge
 	mutex                       sync.Mutex
 }
 
@@ -99,12 +100,15 @@ func NewNginxPlusCollector(nginxClient *plusclient.NginxClient, namespace string
 			"health_checks_fails":     newStreamUpstreamServerMetric(namespace, "health_checks_fails", "Failed health checks"),
 			"health_checks_unhealthy": newStreamUpstreamServerMetric(namespace, "health_checks_unhealthy", "How many times the server became unhealthy (state 'unhealthy')"),
 		},
+		upMetric: newUpMetric(namespace),
 	}
 }
 
 // Describe sends the super-set of all possible descriptors of NGINX Plus metrics
 // to the provided channel.
 func (c *NginxPlusCollector) Describe(ch chan<- *prometheus.Desc) {
+	ch <- c.upMetric.Desc()
+
 	for _, m := range c.totalMetrics {
 		ch <- m
 	}
@@ -135,10 +139,15 @@ func (c *NginxPlusCollector) Collect(ch chan<- prometheus.Metric) {
 
 	stats, err := c.nginxClient.GetStats()
 	if err != nil {
+		c.upMetric.Set(nginxDown)
+		ch <- c.upMetric
 		log.Printf("Error getting stats: %v", err)
 		return
 	}
 
+	c.upMetric.Set(nginxUp)
+	ch <- c.upMetric
+
 	ch <- prometheus.MustNewConstMetric(c.totalMetrics["connections_accepted"],
 		prometheus.CounterValue, float64(stats.Connections.Accepted))
 	ch <- prometheus.MustNewConstMetric(c.totalMetrics["connections_dropped"],