Move queries from queries.yaml to collectors (#801)

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

collector/pg_postmaster.go

@@ -0,0 +1,58 @@
+// Copyright 2023 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"
+)
+
+func init() {
+	registerCollector("postmaster", defaultEnabled, NewPGPostmasterCollector)
+}
+
+type PGPostmasterCollector struct {
+}
+
+func NewPGPostmasterCollector(collectorConfig) (Collector, error) {
+	return &PGPostmasterCollector{}, nil
+}
+
+var (
+	pgPostMasterStartTimeSeconds = prometheus.NewDesc(
+		"pg_postmaster_start_time_seconds",
+		"Time at which postmaster started",
+		[]string{}, nil,
+	)
+
+	pgPostmasterQuery = "SELECT pg_postmaster_start_time from pg_postmaster_start_time();"
+)
+
+func (c *PGPostmasterCollector) Update(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+	row := db.QueryRowContext(ctx,
+		pgPostmasterQuery)
+
+	var startTimeSeconds float64
+	err := row.Scan(&startTimeSeconds)
+	if err != nil {
+		return err
+	}
+	ch <- prometheus.MustNewConstMetric(
+		pgPostMasterStartTimeSeconds,
+		prometheus.GaugeValue, startTimeSeconds,
+	)
+	return nil
+}