Fix up collector registration (#812)

Use const definitions to make collector registration consistent.
* Use collector subsystem name consistently.
* Fix up replication metric name unit.

Signed-off-by: SuperQ <superq@gmail.com>

collector/pg_process_idle.go

@@ -21,16 +21,16 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 )
 
+const processIdleSubsystem = "process_idle"
+
 func init() {
-	registerCollector("statements", defaultEnabled, NewPGProcessIdleCollector)
+	registerCollector(processIdleSubsystem, defaultEnabled, NewPGProcessIdleCollector)
 }
 
 type PGProcessIdleCollector struct {
 	log log.Logger
 }
 
-const processIdleSubsystem = "process_idle"
-
 func NewPGProcessIdleCollector(config collectorConfig) (Collector, error) {
 	return &PGProcessIdleCollector{log: config.logger}, nil
 }