Reseting gauge on metrics

* Reseting gauge metrics

After some value is setted to metricss `postgresql_backends_by_wait_event_type_total`, `postgresql_backends_by_state_total` and `postgresql_locks_total `, the value is not updated if the query returns no rows.

To fix that, the gauge is reseted every time when the query is executed, thus grants that the old value was cleared.

Author: merencia

gauges/backends.go

@@ -56,6 +56,7 @@ func (g *Gauges) BackendsByState() *prometheus.GaugeVec {
 
 	go func() {
 		for {
+			gauge.Reset()
 			var backendsByState []backendsByState
 			if err := g.query(backendsByStateQuery, &backendsByState, emptyParams); err == nil {
 				for _, row := range backendsByState {
@@ -110,6 +111,7 @@ func (g *Gauges) BackendsByWaitEventType() *prometheus.GaugeVec {
 
 	go func() {
 		for {
+			gauge.Reset()
 			var backendsByWaitEventType []backendsByWaitEventType
 			if err := g.query(g.backendsByWaitEventTypeQuery(),
 				&backendsByWaitEventType, emptyParams); err == nil {

gauges/locks.go

@@ -23,6 +23,7 @@ func (g *Gauges) Locks() *prometheus.GaugeVec {
 	)
 	go func() {
 		for {
+			gauge.Reset()
 			var locks []locks
 			if err := g.query(
 				`