Support UTF-8 in metric and label names

[ywwg]

As raised in prometheus/client_ruby#306, we will need some way to encode UTF-8 metric and label names in pushgateway URLs. This may be tricky because the current method does allow for encodings of job names and label values, and any solution would have to be backwards compatible with the current syntax. Thoughtful design work is needed.

See: https://github.com/prometheus/pushgateway#url

Feature request

Use case. Why is this important?

UTF-8 Support is coming to prometheus and we need to cover this usecase.