Move pg_post_master query to collector

Signed-off-by: Felix Yuan <felix.yuan@reddit.com>

Author: Sticksman

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 pgPostmaster = map[string]*prometheus.Desc{
+"start_time_seconds": prometheus.NewDesc(
+"pg_postmaster_start_time_seconds",
+"Time at which postmaster started",
+[]string{"process_name"}, nil,
+),
+}
+
+func (c *PGPostmasterCollector) Update(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+row := db.QueryRowContext(ctx,
+`SELECT
+pg_postmaster_start_time
+from pg_postmaster_start_time();`)
+
+var startTimeSeconds float64
+err := row.Scan(&startTimeSeconds)
+if err != nil {
+return err
+}
+ch <- prometheus.MustNewConstMetric(
+pgPostmaster["start_time_seconds"],
+prometheus.GaugeValue, startTimeSeconds, "postmaster",
+)
+return nil
+}