fix: structured log handler formatting issues

[daniel-sanche]

This PR fixes a number of formatting edge cases around the StructuredLogHandler:

• format strings were not handled properly, with arguments dropped (StructuredLogHandler breaks when args in LogRecord #313)
• line breaks would break the log formatting, preventing it from being parsed properly (Log splits by '\n' in the message when using StructuredLogHandler to write logs in Cloud Run #318)
• custom log formatters could not be applied without breaking the fianl GCP formatting (feat: support custom log formatters attached to handlers #314)

Fixes #314, #318, #313 🦕