Document Kubernetes Kit session size logging

[Legioth]

One useful feature in the session handling logic in Kubernets Kit is that it logs the size and processing time for each serialized session when debug logging is enabled. This should be documented in e.g. https://vaadin.com/docs/latest/tools/kubernetes/session-replication or https://vaadin.com/docs/latest/tools/kubernetes/session-replication-debug-tool.

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[jorgheymans]

If you think this issue is important

Anything that can improve the observability of the (vaadin) session is important 👍