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>

collector/pg_database.go

@@ -0,0 +1,63 @@
+// Copyright 2021 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package collector
+
+import (
+	"context"
+	"database/sql"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type PGDatabaseCollector struct{}
+
+func NewPGDatabaseCollector() *PGDatabaseCollector {
+	return &PGDatabaseCollector{}
+}
+
+var pgDatabase = map[string]*prometheus.Desc{
+	"size_bytes": prometheus.NewDesc(
+		"pg_database_size_bytes",
+		"Disk space used by the database",
+		[]string{"datname"}, nil,
+	),
+}
+
+func (PGDatabaseCollector) Update(ctx context.Context, db *sql.DB, server string) ([]prometheus.Metric, error) {
+	metrics := []prometheus.Metric{}
+	rows, err := db.QueryContext(ctx,
+		`SELECT pg_database.datname
+		,pg_database_size(pg_database.datname)
+		FROM pg_database;`)
+	if err != nil {
+		return metrics, err
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var datname string
+		var size int64
+		if err := rows.Scan(&datname, &size); err != nil {
+			return metrics, err
+		}
+		metrics = append(metrics, prometheus.MustNewConstMetric(
+			pgDatabase["size_bytes"],
+			prometheus.GaugeValue, float64(size), datname,
+		))
+	}
+	if err := rows.Err(); err != nil {
+		return metrics, err
+	}
+	return metrics, nil
+}