perf: avoid copying MetricFamily in Family::Collect

[FujiwaraNaoto]

perf: avoid copying MetricFamily in Family::Collect

Summary

Family<T>::Collect() built the family in a local variable and returned it via
return {family};. A braced-init-list copies its elements, so this deep-copied
the whole metric vector (every ClientMetric and all of its labels) on every
scrape, temporarily requiring twice the memory of the collected family.

The family is now constructed in place inside the returned vector, so no copy is
made. The redundant std::move() around the prvalue returned by
CollectMetric() is dropped as well; a prvalue already binds to
std::vector::push_back(T&&).

Changes

• core/src/family.cc: build the returned std::vector<MetricFamily> in place
  instead of copy-constructing it from a braced-init-list.
• core/src/family.cc: remove the redundant std::move() in the collection loop.

Behavior

No functional change. The returned data is identical; only the number of
allocations and copies performed during a collection is reduced.

Registry::Collect() already moves the families out of Family<T>::Collect()
via std::make_move_iterator, so this removes the last copy on that path.

Relates to #741

In #741 a std::bad_alloc is thrown from the MetricFamily copy constructor
while collecting metrics, which is exactly the copy removed here. Cutting the
peak memory required for a collection makes that failure less likely, but this
change does not add exception handling, so it does not fully implement the
request in #741 to keep the process alive when an allocation fails.

[FujiwaraNaoto]

Could you please review it?

The Bazel CI failure is unrelated to this change — it fails while computing the main repo mapping, before anything is built.

[gjasny]

Thanks for fixing!