PMM-7 Try to fix test

Author: BupycHuk

exporter/diagnostic_data_collector_test.go

@@ -419,6 +419,9 @@ func TestDisconnectedDiagnosticDataCollector(t *testing.T) {
 
 	// The last \n at the end of this string is important
 	expected := strings.NewReader(`
+	# 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="",mongodb="",vendor=""} 1` + "\n")