PMM-11984: Add new metrics pg_available_extensions and pg_extensions to expose info about available extensions and installed extensions accordingly (#117)

* PMM-11984: Add new metrics `pg_available_extensions` and `pg_extensions` to expose info about available extensions and installed extensions accordingly

* PMM-11984: Exclude comment from the metric

* PMM-11984: Fix small issues

* PMM-11984: Update label values

---------

Co-authored-by: Alexey Mukas <91831719+ritbl@users.noreply.github.com>

collector/pg_extensions.go

@@ -0,0 +1,122 @@
+package collector
+
+import (
+	"context"
+	"database/sql"
+	"github.com/go-kit/log"
+	"github.com/prometheus/client_golang/prometheus"
+	"strconv"
+)
+
+func init() {
+	registerCollector("extensions", defaultEnabled, NewExtensionsCollector)
+}
+
+var pgExtensions = map[string]*prometheus.Desc{
+	"pg_available_extensions": prometheus.NewDesc(
+		"pg_available_extensions",
+		"Extensions that are available for installation",
+		[]string{
+			"name",
+			"default_version",
+			"installed_version",
+		},
+		prometheus.Labels{},
+	),
+	"pg_extensions": prometheus.NewDesc(
+		"pg_extensions",
+		"Installed extensions",
+		[]string{
+			"name",
+			"relocatable",
+			"version",
+		},
+		prometheus.Labels{},
+	),
+}
+
+type ExtensionsCollector struct {
+	logger log.Logger
+}
+
+func NewExtensionsCollector(logger log.Logger) (Collector, error) {
+	return &ExtensionsCollector{logger: logger}, nil
+}
+
+func (e *ExtensionsCollector) Update(ctx context.Context, server *server, ch chan<- prometheus.Metric) error {
+	db, err := server.GetDB()
+	if err != nil {
+		return err
+	}
+
+	err = e.scrapeAvailableExtensions(ctx, db, ch)
+	if err != nil {
+		return err
+	}
+
+	err = e.scrapeInstalledExtensions(ctx, db, ch)
+	if err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (e *ExtensionsCollector) scrapeInstalledExtensions(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+	rowsExtensions, err := db.QueryContext(ctx, `SELECT extname, extrelocatable, extversion FROM pg_extension`)
+
+	if err != nil {
+		return err
+	}
+	defer rowsExtensions.Close()
+
+	for rowsExtensions.Next() {
+		var extname string
+		var extrelocatable bool
+		var extversion string
+		if err := rowsExtensions.Scan(&extname, &extrelocatable, &extversion); err != nil {
+			return err
+		}
+
+		ch <- prometheus.MustNewConstMetric(
+			pgExtensions["pg_extensions"],
+			prometheus.GaugeValue,
+			1,
+			extname,
+			strconv.FormatBool(extrelocatable),
+			extversion,
+		)
+	}
+
+	return nil
+}
+
+func (e *ExtensionsCollector) scrapeAvailableExtensions(ctx context.Context, db *sql.DB, ch chan<- prometheus.Metric) error {
+	rows, err := db.QueryContext(ctx, `SELECT name, default_version, installed_version FROM pg_available_extensions`)
+	if err != nil {
+		return err
+	}
+	defer rows.Close()
+
+	for rows.Next() {
+		var name sql.NullString
+		var defaultVersion sql.NullString
+		var installedVersion sql.NullString
+		if err := rows.Scan(&name, &defaultVersion, &installedVersion); err != nil {
+			return err
+		}
+
+		ch <- prometheus.MustNewConstMetric(
+			pgExtensions["pg_available_extensions"],
+			prometheus.GaugeValue,
+			1,
+			name.String,
+			defaultVersion.String,
+			installedVersion.String,
+		)
+	}
+
+	return nil
+}
+
+var _ = (Collector)(&ExtensionsCollector{})