UTF8 Label Encoding

How should we encode utf8 labels?

if self.prometheus_full_utf8 {
    write!(&mut key_encoder, "{}", key)?;
} else {
    write!(&mut key_encoder, "{}", escape_label(key.as_str()))?;
}

I feel like doing something like this is less than ideal & that the key encoder itself should should either escape or not depending on the encoding (text or protobuf).

--

# HELP target_info Information about the target
# TYPE target_info info
target_info_info{service.name="xxx"} 1
# EOF

Start with the shown key_encoder path and compare the prometheus_full_utf8 branches, including escape_label and the text/protobuf encoding distinction. Determine the intended UTF-8 label encoding behavior and how it should apply across encodings; done means the project has a decided, consistent approach for the target_info example.