Use of auto that causes a copy

Unnecessary object copies can affect performance. Using the auto keyword without an & causes a copy.
/data/mci/22bd5cb6273c9c571686198c9f6023af/bazel_cache/3d8c86e2cd3c28a487a62d019e05fed8/execroot/main/src/mongo/db/pipeline/document_source_hybrid_scoring_util.cpp:234: AUTO_CAUSES_COPY 174805 Using the "auto" keyword without an "&" causes the copy of an object of type "std::_Rb_tree<std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::_Identity<std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::less<std::cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > >, std::allocator<std::_cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > >::value_type".
/data/mci/22bd5cb6273c9c571686198c9f6023af/bazel_cache/3d8c86e2cd3c28a487a62d019e05fed8/execroot/_main/src/mongo/db/pipeline/document_source_hybrid_scoring_util.cpp:234: AUTO_CAUSES_COPY 174805 The variable "fieldName" has a non-"const" type, however its value is never changed.