export metrics to track shared preload libraries in postgres (#8)

exports one new metric,
`pg_shared_preload_libraries_library_enabled{library=<name}=1` per
library across the fleet

This helps us understand and track rollouts

Author: tom-pang

collector/pg_shared_preload_libraries.go

@@ -0,0 +1,87 @@
+// Copyright 2025 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"
+	"sort"
+	"strings"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+const sharedPreloadLibrariesSubsystem = "settings"
+
+func init() {
+	registerCollector(sharedPreloadLibrariesSubsystem, defaultEnabled, NewPGSharedPreloadLibrariesCollector)
+}
+
+type PGSharedPreloadLibrariesCollector struct{}
+
+func NewPGSharedPreloadLibrariesCollector(collectorConfig) (Collector, error) {
+	return &PGSharedPreloadLibrariesCollector{}, nil
+}
+
+var (
+	pgSharedPreloadLibrariesLibraryEnabled = prometheus.NewDesc(
+		prometheus.BuildFQName(
+			namespace,
+			sharedPreloadLibrariesSubsystem,
+			"shared_preload_library_enabled",
+		),
+		"Whether a library is listed in shared_preload_libraries (1=yes).",
+		[]string{"library"}, nil,
+	)
+
+	pgSharedPreloadLibrariesQuery = "SELECT setting FROM pg_settings WHERE name = 'shared_preload_libraries'"
+)
+
+func (c *PGSharedPreloadLibrariesCollector) Update(ctx context.Context, instance *Instance, ch chan<- prometheus.Metric) error {
+	db := instance.getDB()
+	row := db.QueryRowContext(ctx, pgSharedPreloadLibrariesQuery)
+
+	var setting sql.NullString
+	err := row.Scan(&setting)
+	if err != nil {
+		return err
+	}
+
+	// Parse, trim, dedupe and sort libraries for stable series emission.
+	libsSet := map[string]struct{}{}
+	if setting.Valid && setting.String != "" {
+		for _, raw := range strings.Split(setting.String, ",") {
+			lib := strings.TrimSpace(raw)
+			if lib == "" {
+				continue
+			}
+			libsSet[lib] = struct{}{}
+		}
+	}
+	libs := make([]string, 0, len(libsSet))
+	for lib := range libsSet {
+		libs = append(libs, lib)
+	}
+	sort.Strings(libs)
+
+	for _, lib := range libs {
+		ch <- prometheus.MustNewConstMetric(
+			pgSharedPreloadLibrariesLibraryEnabled,
+			prometheus.GaugeValue,
+			1,
+			lib,
+		)
+	}
+	return nil
+}

collector/pg_shared_preload_libraries_test.go

@@ -0,0 +1,187 @@
+// Copyright 2025 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"
+	"testing"
+
+	"github.com/DATA-DOG/go-sqlmock"
+	"github.com/prometheus/client_golang/prometheus"
+	dto "github.com/prometheus/client_model/go"
+	"github.com/smartystreets/goconvey/convey"
+)
+
+func TestPGSharedPreloadLibrariesCollector(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub database connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &Instance{db: db}
+
+	columns := []string{"setting"}
+	rows := sqlmock.NewRows(columns).
+		AddRow("pg_stat_statements, auto_explain, pg_hint_plan")
+
+	mock.ExpectQuery(sanitizeQuery(pgSharedPreloadLibrariesQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGSharedPreloadLibrariesCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGSharedPreloadLibrariesCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{
+		// Emitted in sorted order: auto_explain, pg_hint_plan, pg_stat_statements
+		{labels: labelMap{"library": "auto_explain"}, value: 1, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"library": "pg_hint_plan"}, value: 1, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"library": "pg_stat_statements"}, value: 1, metricType: dto.MetricType_GAUGE},
+	}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled expectations: %s", err)
+	}
+}
+
+func TestPGSharedPreloadLibrariesCollectorEmpty(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub database connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &Instance{db: db}
+
+	columns := []string{"setting"}
+	rows := sqlmock.NewRows(columns).
+		AddRow("")
+
+	mock.ExpectQuery(sanitizeQuery(pgSharedPreloadLibrariesQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGSharedPreloadLibrariesCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGSharedPreloadLibrariesCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled expectations: %s", err)
+	}
+}
+
+func TestPGSharedPreloadLibrariesCollectorSingle(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub database connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &Instance{db: db}
+
+	columns := []string{"setting"}
+	rows := sqlmock.NewRows(columns).
+		AddRow("pg_stat_statements")
+
+	mock.ExpectQuery(sanitizeQuery(pgSharedPreloadLibrariesQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGSharedPreloadLibrariesCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGSharedPreloadLibrariesCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{
+		{labels: labelMap{"library": "pg_stat_statements"}, value: 1, metricType: dto.MetricType_GAUGE},
+	}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled expectations: %s", err)
+	}
+}
+
+func TestPGSharedPreloadLibrariesCollectorWhitespaceAndDuplicates(t *testing.T) {
+	db, mock, err := sqlmock.New()
+	if err != nil {
+		t.Fatalf("Error opening a stub database connection: %s", err)
+	}
+	defer db.Close()
+
+	inst := &Instance{db: db}
+
+	columns := []string{"setting"}
+	rows := sqlmock.NewRows(columns).
+		AddRow("pg_stat_statements,  auto_explain,  pg_hint_plan , auto_explain , pg_stat_statements ")
+
+	mock.ExpectQuery(sanitizeQuery(pgSharedPreloadLibrariesQuery)).WillReturnRows(rows)
+
+	ch := make(chan prometheus.Metric)
+	go func() {
+		defer close(ch)
+		c := PGSharedPreloadLibrariesCollector{}
+
+		if err := c.Update(context.Background(), inst, ch); err != nil {
+			t.Errorf("Error calling PGSharedPreloadLibrariesCollector.Update: %s", err)
+		}
+	}()
+
+	expected := []MetricResult{
+		{labels: labelMap{"library": "auto_explain"}, value: 1, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"library": "pg_hint_plan"}, value: 1, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"library": "pg_stat_statements"}, value: 1, metricType: dto.MetricType_GAUGE},
+	}
+
+	convey.Convey("Metrics comparison", t, func() {
+		for _, expect := range expected {
+			m := readMetric(<-ch)
+			convey.So(expect, convey.ShouldResemble, m)
+		}
+	})
+	if err := mock.ExpectationsWereMet(); err != nil {
+		t.Errorf("there were unfulfilled expectations: %s", err)
+	}
+}