Require that metrics be grouped together for text format.

While it'd be nice for TYPE to be first,
that's not what the examples do so we leave the ordering
of HELP/TYPE unspecified.

content/docs/instrumenting/exposition_formats.md

@@ -82,6 +82,9 @@ syntax (EBNF):
 `metric_name` and `label_name` have the usual Prometheus expression language restrictions. `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.
 `value` is a float, and timestamp an `int64` (milliseconds since epoch, i.e. 1970-01-01 00:00:00 UTC, excluding leap seconds), represented as required by the [Go strconv package](http://golang.org/pkg/strconv/) (see functions `ParseInt` and `ParseFloat`). In particular, `Nan`, `+Inf`, and `-Inf` are valid values.
 
+All lines for a given metric should be provided as one uninterrupted group, with
+the optional `HELP` and `TYPE` lines first.
+
 The `histogram` and `summary` types are difficult to represent in the text
 format. The following conventions apply: