Allow ipv6 scope syntax in a serial ID

[jaeheon]

Without escaping, the test simulates the logger logic and issues this error: E Exception: Error occurred: unsupported format character ']' (0x5d) at index 14

With escaping, tox passes all tests. It's a bit unfortunate that the debug_tag_ is escaped directly, instead of at its injection point into the custom logger's tag field.

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This change is