fix: Ignore frozen hashes when formatting JSON log messages

The JsonLogger does in place updates to Time values, which does not play well with frozen hashes. Let's keep them untouched.

This will help fixing the issue raised in incident inc-6704.

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My first idea was to handle frozen hashes, here is the MR: !224 (closed).

However, after using this script to roughly analyze memory allocations, it has shown 30% more in the worst case. In practice, it wouldn't be so bad to deal with small hash values, especially non-frozen values, but I did not want to risk it and went with a simpler and obvious approach.