Implement changes needed to support UTF8 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:

• optionally skip name validation depending on negotiated version.

More tasks may be added as needed.

[ywwg]

This is a noop for handling validation itself, but work will be needed here to handle the content negotiation and what format to serve depending on that

[ywwg]

It looks like almost nothing needs to be done to the agent other than setting the appropriate library values.
Draft: #1431

[ywwg]

all diffs have vanished, no changes needed

[ywwg]

reopening -- the content negotiation changes break some tests and will need fixing

[ywwg]

fixed now #1448