Conversion back and forth between MetricFamily protobuf and text format.

The idea here is to always go via the protobufs if dealing with the
text format. That won't always be the most efficient way, but it
avoids the multiplicity of conversion routines required for direct
conversion (e.g. text format -> internal representation in the
Prometheus server). The loss of efficiency is acceptable because the
text format should not be used in high performance (high throughput,
low latency) situations anyway.

In that way, the text format stays perfectly isolated from other parts
of the code. To receive text format, just plug the conversion in
before the code path that normally reads protobufs. Correspondingly,
for sending text format, simply replace the WriteDelimited call by a
text.Create call.

Nevertheless, the conversion code itself is optimized for efficiency
and minimized memory churn (which was one of the reason for handcoding
the parser and not using a lexer/parser code generation tool).

Change-Id: Iee45ffe8aa421a844225d13a1f859becd8a3b066