span type/subtype : add app/handler

[SylvainJuge]

Adding app/handler to accomodate Javalin plugin which is currently using it.

This is a temporary workaround and should be later be removed through #502

For example, it would likely be merged in a generic app/internal for internal spans for alignment.

[apmmachine]

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• Start Time: 2021-09-08T11:33:49.801+0000

• Duration: 3 min 5 sec

• Commit: 43b6026

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