PMM-11247: Do not create a metric mongodb_mongod_storage_engine if it's not possible to retrieve (#612)

* PMM-11247: Do not create a metric mongodb_mongod_storage_engine if it's not possible to retrieve

For example, we can't get the type of engine for mongos instance. So in this case we can skip creating this metric.

* PMM-11247: Ignore lint issue

Author: dliakhov

exporter/diagnostic_data_collector_test.go

@@ -96,6 +96,9 @@ func TestDiagnosticDataCollectorWithCompatibleMode(t *testing.T) {
 
 	// The last \n at the end of this string is important
 	expected := strings.NewReader(fmt.Sprintf(`
+	# HELP mongodb_mongod_storage_engine The storage engine used by the MongoDB instance
+	# TYPE mongodb_mongod_storage_engine gauge
+	mongodb_mongod_storage_engine{engine="wiredTiger"} 1
 	# HELP mongodb_version_info The server version
 	# TYPE mongodb_version_info gauge
 	mongodb_version_info{edition="Community",mongodb="%s",vendor="%s"} 1`, version, vendor) + "\n")
@@ -105,6 +108,7 @@ func TestDiagnosticDataCollectorWithCompatibleMode(t *testing.T) {
 	// 2. We need to check against know values. Don't use metrics that return counters like uptime
 	//    or counters like the number of transactions because they won't return a known value to compare
 	filter := []string{
+		"mongodb_mongod_storage_engine",
 		"mongodb_version_info",
 	}
 
@@ -287,9 +291,6 @@ func TestDisconnectedDiagnosticDataCollector(t *testing.T) {
 	# HELP mongodb_mongod_replset_my_state An integer between 0 and 10 that represents the replica state of the current member
 	# TYPE mongodb_mongod_replset_my_state gauge
 	mongodb_mongod_replset_my_state{set=""} 6
-	# HELP mongodb_mongod_storage_engine The storage engine used by the MongoDB instance
-	# TYPE mongodb_mongod_storage_engine gauge
-	mongodb_mongod_storage_engine{engine="Engine is unavailable"} 1
 	# HELP mongodb_version_info The server version
 	# TYPE mongodb_version_info gauge
 	mongodb_version_info{edition="",mongodb="",vendor=""} 1` + "\n")

exporter/v1_compatibility.go

@@ -798,7 +798,11 @@ func specialMetrics(ctx context.Context, client *mongo.Client, m bson.M, l *logr
 		l.Errorf("cannot retrieve MongoDB buildInfo: %s", err)
 	}
 
-	metrics = append(metrics, storageEngine(m))
+	if engine, err := storageEngine(m); err != nil {
+		l.Errorf("cannot retrieve engine type: %s", err)
+	} else {
+		metrics = append(metrics, engine)
+	}
 	metrics = append(metrics, serverVersion(buildInfo))
 	metrics = append(metrics, myState(ctx, client))
 
@@ -853,21 +857,23 @@ func retrieveMongoDBBuildInfo(ctx context.Context, client *mongo.Client, l *logr
 	return bi, nil
 }
 
-func storageEngine(m bson.M) prometheus.Metric {
+func storageEngine(m bson.M) (prometheus.Metric, error) { //nolint:ireturn
 	v := walkTo(m, []string{"serverStatus", "storageEngine", "name"})
 	name := "mongodb_mongod_storage_engine"
 	help := "The storage engine used by the MongoDB instance"
 
 	engine, ok := v.(string)
 	if !ok {
-		engine = "Engine is unavailable"
+		return nil, errors.New("Engine is unavailable")
 	}
 	labels := map[string]string{"engine": engine}
 
 	d := prometheus.NewDesc(name, help, nil, labels)
-	metric, _ := prometheus.NewConstMetric(d, prometheus.GaugeValue, float64(1))
-
-	return metric
+	metric, err := prometheus.NewConstMetric(d, prometheus.GaugeValue, float64(1))
+	if err != nil {
+		return nil, errors.Wrap(err, "cannot create metric for engine type")
+	}
+	return metric, nil
 }
 
 func serverVersion(bi buildInfo) prometheus.Metric { //nolint:ireturn