Rename the last_scrape_connection_error metric to up.

On advice from @SuperQ this is inline with the convention used in similar
exporters (example being the mysqld exporter). The full metric name is thus
`pg_up`.

Closes #135

postgres_exporter.go

@@ -668,7 +668,7 @@ type Exporter struct {
 	userQueriesPath  string
 	duration         prometheus.Gauge
 	error            prometheus.Gauge
-	connectionError  prometheus.Gauge
+	psqlUp           prometheus.Gauge
 	userQueriesError *prometheus.GaugeVec
 	totalScrapes     prometheus.Counter
 
@@ -711,11 +711,10 @@ func NewExporter(dsn string, userQueriesPath string) *Exporter {
 			Name:      "last_scrape_error",
 			Help:      "Whether the last scrape of metrics from PostgreSQL resulted in an error (1 for error, 0 for success).",
 		}),
-		connectionError: prometheus.NewGauge(prometheus.GaugeOpts{
+		psqlUp: prometheus.NewGauge(prometheus.GaugeOpts{
 			Namespace: namespace,
-			Subsystem: exporter,
-			Name:      "last_scrape_connection_error",
-			Help:      "Whether the last scrape of metrics from PostgreSQL was able to connect to the server (1 for error, 0 for success).",
+			Name:      "up",
+			Help:      "Whether the last scrape of metrics from PostgreSQL was able to connect to the server (1 for yes, 0 for no).",
 		}),
 		userQueriesError: prometheus.NewGaugeVec(prometheus.GaugeOpts{
 			Namespace: namespace,
@@ -763,6 +762,7 @@ func (e *Exporter) Collect(ch chan<- prometheus.Metric) {
 	ch <- e.duration
 	ch <- e.totalScrapes
 	ch <- e.error
+	ch <- e.psqlUp
 	e.userQueriesError.Collect(ch)
 }
 
@@ -969,15 +969,15 @@ func (e *Exporter) getDB(conn string) (*sql.DB, error) {
 	if e.dbConnection == nil {
 		d, err := sql.Open("postgres", conn)
 		if err != nil {
-			e.connectionError.Set(1)
+			e.psqlUp.Set(0)
 			return nil, err
 		}
 		err = d.Ping()
 		if err != nil {
-			e.connectionError.Set(1)
+			e.psqlUp.Set(0)
 			return nil, err
 		}
-		e.connectionError.Set(0)
+		e.psqlUp.Set(1)
 
 		d.SetMaxOpenConns(1)
 		d.SetMaxIdleConns(1)