CosmosStore: Log byte counts for unfolds

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Especially when writing, though it's also relatively useful when reading Tip and/or any time we emit event byte counts either

Ideally, it would even be able to log the compressed sizes, but even having the basic logging for the Sync call include this alongside the (already supplied) bytes for the events is very useful for troubleshooting

Note #305 alters logging to only emit byte counts when Debug log level is enabled

Any impl should follow the structures used in DynamoStore, which implements the bulk of this