You can use custom services for this. You create a custom service on the class and the method and you should be able to see the throughput/count for the service method. See more on this here
Is it ok to create a custom service eventhough the service has already been recognized by Dynatrace out-of-the-box. The link that you provided mentionned that it can be used to define entry points...
That is correct. This will break the class and method into its own separate custom service and you will be able to see executions of this method as a service method/request throughput within this new custom service. If you will like to keep it within the out of the box service, you can go the Request attribute route explained here.
In general you have to create Request Attribute. You can use it not for data extraction but the same as Appmon custom sensor. All you have to do is to pick amount of executions (for example) as attribute value. On purepaths this method will be measured. You will also be able to filter transactions by this attribute. In result you will know all endpoint that are using it.