Consider making SeverityNumber an IntEnum #5785

Before stabilizing opentelemetry.logs, we should consider changing SeverityNumber from enum.Enum to enum.IntEnum:

1. Ergonomics: It allows comparing enum values directly (e.g. severity_number < SeverityNumber.INFO in enabled() checks) without having to do .value on both sides. The type is already called SeverityNumber, so it's confusing that it isn't an IntEnum / comparable as a number.
2. Performance: This came up while benchmarking the logging hot path for bottlenecks. On enum.Enum, .value is a descriptor lookup in Python, so comparing a.value >= b.value takes ~380 ns vs ~97 ns for a >= b with IntEnum (~4x faster). It also avoids getattr(record.severity_number, "value", None) in the OTLP encoder.
3. Timing: IntEnum still supports .value and .name, but changing Enum to IntEnum after 1.0 would change isinstance(sev, int) and sev == 9 from False to True, so it's best to do before stabilizing opentelemetry.logs.