Do not generate XXX fields in protobufs (#1725)

* Do not generate XXX fields in protobufs

We don't need them. By gettig rid of them we achieve:

- smaller memory footprint
- smaller payloads on the wire
- same in-memory layout with core Prometheus structs opening path for all sorts of optimisations such as unsafe casts, leading to better memory use and faster :)
- see related issue: prometheus/prometheus#6029

The tests show SG memory allocations dropped 5% for query execution:
- Showing nodes accounting for -359.20MB, 6.19% of 5798.93MB total

Signed-off-by: Philip Panyukov <ppanyukov@googlemail.com>

* comment to explain why do don't generate XXX fields in protobufs

Signed-off-by: Philip Panyukov <ppanyukov@googlemail.com>