Add database connection limits metrics

Signed-off-by: Jocelyn Thode <[email protected]>

Author: jocelynthode

collector/pg_database.go

@@ -53,12 +53,22 @@ var (
 		"Disk space used by the database",
 		[]string{"datname"}, nil,
 	)
+	pgDatabaseConnectionLimitsDesc = prometheus.NewDesc(
+		prometheus.BuildFQName(
+			namespace,
+			databaseSubsystem,
+			"connection_limit",
+		),
+		"Connection limit set for the database",
+		[]string{"datname"}, nil,
+	)
 
-	pgDatabaseQuery     = "SELECT pg_database.datname FROM pg_database;"
-	pgDatabaseSizeQuery = "SELECT pg_database_size($1)"
+	pgDatabaseQuery                 = "SELECT pg_database.datname FROM pg_database;"
+	pgDatabaseSizeQuery             = "SELECT pg_database_size($1)"
+	pgDatabaseConnectionLimitsQuery = "select pg_database.datconnlimit FROM pg_database WHERE pg_database.datname = $1"
 )
 
-// Update implements Collector and exposes database size.
+// Update implements Collector and exposes database size and connection limits.
 // It is called by the Prometheus registry when collecting metrics.
 // The list of databases is retrieved from pg_database and filtered
 // by the excludeDatabase config parameter. The tradeoff here is that
@@ -114,6 +124,20 @@ func (c PGDatabaseCollector) Update(ctx context.Context, instance *instance, ch
 			pgDatabaseSizeDesc,
 			prometheus.GaugeValue, sizeMetric, datname,
 		)
+
+		var connLimit sql.NullInt64
+		err = db.QueryRowContext(ctx, pgDatabaseConnectionLimitsQuery, datname).Scan(&connLimit)
+		if err != nil {
+			return err
+		}
+		connLimitMetric := 0.0
+		if connLimit.Valid {
+			connLimitMetric = float64(connLimit.Int64)
+		}
+		ch <- prometheus.MustNewConstMetric(
+			pgDatabaseConnectionLimitsDesc,
+			prometheus.GaugeValue, connLimitMetric, datname,
+		)
 	}
 	return rows.Err()
 }

collector/pg_database_test.go

@@ -37,6 +37,9 @@ func TestPGDatabaseCollector(t *testing.T) {
 	mock.ExpectQuery(sanitizeQuery(pgDatabaseSizeQuery)).WithArgs("postgres").WillReturnRows(sqlmock.NewRows([]string{"pg_database_size"}).
 		AddRow(1024))
 
+	mock.ExpectQuery(sanitizeQuery(pgDatabaseConnectionLimitsQuery)).WithArgs("postgres").WillReturnRows(sqlmock.NewRows([]string{"datconnlimit"}).
+		AddRow(15))
+
 	ch := make(chan prometheus.Metric)
 	go func() {
 		defer close(ch)
@@ -48,6 +51,7 @@ func TestPGDatabaseCollector(t *testing.T) {
 
 	expected := []MetricResult{
 		{labels: labelMap{"datname": "postgres"}, value: 1024, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"datname": "postgres"}, value: 15, metricType: dto.MetricType_GAUGE},
 	}
 	convey.Convey("Metrics comparison", t, func() {
 		for _, expect := range expected {
@@ -77,6 +81,9 @@ func TestPGDatabaseCollectorNullMetric(t *testing.T) {
 	mock.ExpectQuery(sanitizeQuery(pgDatabaseSizeQuery)).WithArgs("postgres").WillReturnRows(sqlmock.NewRows([]string{"pg_database_size"}).
 		AddRow(nil))
 
+	mock.ExpectQuery(sanitizeQuery(pgDatabaseConnectionLimitsQuery)).WithArgs("postgres").WillReturnRows(sqlmock.NewRows([]string{"datconnlimit"}).
+		AddRow(nil))
+
 	ch := make(chan prometheus.Metric)
 	go func() {
 		defer close(ch)
@@ -88,6 +95,7 @@ func TestPGDatabaseCollectorNullMetric(t *testing.T) {
 
 	expected := []MetricResult{
 		{labels: labelMap{"datname": "postgres"}, value: 0, metricType: dto.MetricType_GAUGE},
+		{labels: labelMap{"datname": "postgres"}, value: 0, metricType: dto.MetricType_GAUGE},
 	}
 	convey.Convey("Metrics comparison", t, func() {
 		for _, expect := range expected {