def TNEE_metric(bpmn_graph):
return get_nodes_count(bpmn_graph, node_type='<STR_LIT>')<EOL>
Returns the value of the TNEE metric (Total Number of End Events of the Model)
for the BPMNDiagramGraph instance.

:param bpmn_graph: an instance of BpmnDiagramGraph representing BPMN model.