axes should provide strong exception guarantee for metadata

[HDembinski]

template <class It, class = detail::requires_iterator<It>> variable(It begin, It end, metadata_type meta = {}, options_type options = {}, allocator_type alloc = {}) does not provide strong exception guarantee: if user writes variable(it_begin, it_end, std::move(str)) and constructor body throws then the value inside str is lost (the value was moved into parameter and then data member before the exception). So one can not write while(std::cin>>str>>...){ try{ variable(..., ..., str); ... }catch(...){} } and has to take on the burden of saving the str elsewhere (so the user do not have to type it again) when other constructor arguments might be illegal and have to be typed again.

Originally posted by @jhcarl0814 in https://github.com/boostorg/histogram/issues/372#issuecomment-1364386000