Add pg_database collector

Converts the pg_database metrics from queries.yaml to a built in collector. This is enabled by default because it is not likely to be a performance problem and likely very useful data.

Signed-off-by: Joe Adams <github@joeadams.io>

cmd/postgres_exporter/server.go

@@ -14,13 +14,16 @@
 package main
 
 import (
+	"context"
 	"database/sql"
 	"fmt"
+	"log"
 	"sync"
 	"time"
 
 	"github.com/blang/semver"
 	"github.com/go-kit/log/level"
+	"github.com/prometheus-community/postgres_exporter/collector"
 	"github.com/prometheus/client_golang/prometheus"
 )
 
@@ -128,6 +131,17 @@ func (s *Server) Scrape(ch chan<- prometheus.Metric, disableSettingsMetrics bool
 		err = fmt.Errorf("queryNamespaceMappings returned %d errors", len(errMap))
 	}
 
+	{
+		pgdb := collector.NewPGDatabaseCollector()
+		metrics, err := pgdb.Update(context.TODO(), s.db, s.String())
+		if err != nil {
+			log.Printf("Failed to scrape pg_database metrics: %s", err)
+		}
+		for _, m := range metrics {
+			ch <- m
+		}
+	}
+
 	return err
 }