Unify escaping

[brian-brazil]

Currently there's slightly different escaping rules for label values and help string. It's not unlikely that implementors will get this incorrect, so we should unify on just one approach.

prometheus/docs#550

[mattbostock]

For convenience, from https://prometheus.io/docs/instrumenting/exposition_formats/#text-format-details:

label_value can be any sequence of UTF-8 characters, but the backslash, the double-quote, and the line-feed characters have to be escaped as \, ", and \n, respectively.

HELP lines may contain any sequence of UTF-8 characters (after the metric name), but the backslash and the line-feed characters have to be escaped as \ and \n, respectively.

Do we want double quotes to be escaped as \" in the HELP lines?

[brian-brazil]

That would be the simplest way to handle it.