Fix exclude-databases for collector package

The pg_database collector was not respecting the --exclude-databases flag and causing problems where databases were not accessible. This now respects the list of databases to exclude.

- Adjusts the Collector create func to take a config struct instead of a logger. This allows more changes like this in the future. I figured we would need to do this at some point but I wasn't sure if we could hold off.
- Split the database size collection to a separate query when database is not excluded.
- Comment some probe code that was not useful/accurate

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

collector/pg_database.go

@@ -26,11 +26,19 @@ func init() {
 }
 
 type PGDatabaseCollector struct {
-	log log.Logger
+	log               log.Logger
+	excludedDatabases []string
 }
 
-func NewPGDatabaseCollector(logger log.Logger) (Collector, error) {
-	return &PGDatabaseCollector{log: logger}, nil
+func NewPGDatabaseCollector(config collectorConfig) (Collector, error) {
+	exclude := config.excludeDatabases
+	if exclude == nil {
+		exclude = []string{}
+	}
+	return &PGDatabaseCollector{
+		log:               config.logger,
+		excludedDatabases: exclude,
+	}, nil
 }
 
 var pgDatabase = map[string]*prometheus.Desc{
@@ -41,20 +49,49 @@ var pgDatabase = map[string]*prometheus.Desc{
 	),
 }
 
-func (PGDatabaseCollector) Update(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+// Update implements Collector and exposes database size.
+// It is called by the Prometheus registry when collecting metrics.
+// The list of databases is retrieved from pg_database and filtered
+// by the excludeDatabase config parameter. The tradeoff here is that
+// we have to query the list of databases and then query the size of
+// each database individually. This is because we can't filter the
+// list of databases in the query because the list of excluded
+// databases is dynamic.
+func (c PGDatabaseCollector) Update(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+	// Query the list of databases
 	rows, err := db.QueryContext(ctx,
 		`SELECT pg_database.datname
-		,pg_database_size(pg_database.datname)
-		FROM pg_database;`)
+		FROM pg_database;
+		`,
+	)
 	if err != nil {
 		return err
 	}
 	defer rows.Close()
 
+	var databases []string
+
 	for rows.Next() {
 		var datname string
+		if err := rows.Scan(&datname); err != nil {
+			return err
+		}
+
+		// Ignore excluded databases
+		// Filtering is done here instead of in the query to avoid
+		// a complicated NOT IN query with a variable number of parameters
+		if sliceContains(c.excludedDatabases, datname) {
+			continue
+		}
+
+		databases = append(databases, datname)
+	}
+
+	// Query the size of the databases
+	for _, datname := range databases {
 		var size int64
-		if err := rows.Scan(&datname, &size); err != nil {
+		err = db.QueryRowContext(ctx, "SELECT pg_database_size($1)", datname).Scan(&size)
+		if err != nil {
 			return err
 		}
 
@@ -68,3 +105,12 @@ func (PGDatabaseCollector) Update(ctx context.Context, db *sql.DB, ch chan<- pro
 	}
 	return nil
 }
+
+func sliceContains(slice []string, s string) bool {
+	for _, item := range slice {
+		if item == s {
+			return true
+		}
+	}
+	return false
+}