Extend Prometheus grammar to cover more of the exposition format

[wandernauta]

This extends the Prometheus grammar so that it can correctly parse the example from the documentation, and adds a number of test cases.

The grammar now...

• Allows spaces ("tokens can be separated by any number of blanks and/or tabs").
• Allows empty lines ("Empty lines are ignored").
• Allows special floating point values inf (infinity) and nan.
• Allows supplying a timestamp field, which is ignored.
• Allows trailing comma in sets of labels.
• Uses metric name and label name regexes from the documentation, so e.g. 123test is no longer allowed.
• Should no longer be confused by backslash escapes. (The escapes themselves are not unescaped.)

Some names in the PEG grammar are aligned with the EBNF grammar in the docs.

Finally, entirely empty (no lines at all) responses are now allowed, to be compatible with upstream Prometheus behavior.