Implement changes needed to support UTF-8 metric and label names in Prometheus

[ywwg]

ref: https://github.com/prometheus/proposals/blob/main/proposals/2023-08-21-utf8.md

Some changes are needed in this library to support the utf8 proposal:

• openmetrics_create needs to understand and construct new syntax (POC exists) UTF-8 support in validation, and some parsers and formatters #537
• library needs to support either legacy mode or utf8 mode, with a way to check for either as well. UTF-8 support in validation, and some parsers and formatters #537
• Update expfmt/text_parse.go to support the new UTF-8 syntax #554
• UTF8: Content negotiation (encoding side) ywwg/common#2

More tasks will be added as code comes together.

[ywwg]

this is done!