Common Trace Format

There's no current specified charset for the metadata packets payload. This can be problematic because if a tracer makes this Unicode and we read it thinking it's ASCII, the displayed text will be weird and the reading could even break.

• lock it in the specifications

• UTF-8: ASCII-compatible, so this shouldn't make a big difference for most cases but allows for i18n of event names and so on
• ASCII: simple, but only English
• avoid everything else IMO

• add a charset byte within the metadata packet header (e.g. 0 => ASCII, 1 => UTF-8, etc.), but this breaks binary compatibility