What are benefits of using third party app metrics tools versus custom build tools?
A third party app metrics tools is great if it already covers all (or most of) the parameters you want to monitor and it presents the information in a convenient enough manner for your use case - you just need to (find it first and then) learn how to install/use/maintain it and then do that.

Advantages:


avoids the software development and maintenance costs, which can be quite high depending on the capabilities
can be immediately used, a custom tool needs to be developed first
technical support (not always great, tho)
standardisation: if the tool is a popular one - expertise on it can be requested on a job description instead of having to train every user on it


However there are many cases in which custom development is needed, some already mentioned by Michael: reliability, security/confidentiality, lack of integrations for your custom applications.

Depending on each particular case, custom development can go from tiny scripts extending the coverage of a third party tool to complex wrappers assembling multiple such tools to achieve the necessary monitoring capabilities

Such development is not always lightweight:


it may require in-depth tool knowledge and/or language expertise outside of your team
it may need management for heavy/complex or even conflicting dependencies
the adaptation layers for the tool's actual usage environment may be very heavy


Sometimes the cost of such development is higher overall than that of developing a custom system.

Othertimes the functionality actually needed from the third party tools is a relatively small percent of its capability set and the overhead for learning/installing/operating the tool is simply not worthy.

You guessed it - I wrote my own monitoring tool :)