allow for visibility on the session serialization

[jorgheymans]

For applications with complex screens, it's advisable to keep an eye on the session size and what ends up in the session. Currently, a developer has no visibility on this. If we could hook up a micrometer meter of some sort to the serialization mechanism, then we can make the results of the session serde visible in a graph and action on it. The size of the serialized object stream is useful to know, the number of objects as well. kubernetes-kit should not have a dependency on micrometer, but perhaps the SessionSerializationCallback could be extended with a methods like onSerializationSuccess(long numberOfBytes, long numberOfObjects) ? Can this be done without enable serde debug, i.e. on a production instance?

Another type of visibility is knowing what is contained in the session. Is it feasible to have a debug mechanism that would allow a developer to dump the complete serialized object graph in text format to a file, and verify if there are not any objects there that should not be. It's very easy to make such mistakes during screen development.