This cleanups up the toString() methods of all implementations of
these interfaces, as pre-work for adding tests against the legacy
logging statement in FlowRunner used for ICANN reporting, so that we
can validate against any changes to that log statement in the future.
It removes system hash codes since those aren't really safe to rely on
in test code and they really don't help with debugging anyway. It
also standardizes SessionMetadata.toString() a bit and regroups
methods on that interface so all the getters are together.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125686039
TransportCredentials are per-request, not per-session, and
there's no reason to carry them within SessionMetadata.
While I'm in here, get rid of "null" credentials.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=125202213
