Several of the change stream execution stages and helper files read fields from the oplog entry by specifying the field names as character literals.

Example from change_stream_event_transform.cpp:

            if (auto nssField = oField.getField("drop"); !nssField.missing()) {
...
            } else if (auto nssField = oField.getField("renameCollection"); !nssField.missing()) {
...

This works, but we should replace the character literal field names with some symbolic constants, so they can be easier found by code analysis (cross-references) and tools (e.g. grep.

The following files should be checked and improved if possible:

• src/mongo/db/pipeline/change_stream_event_transform.cpp
• src/mongo/db/exec/agg/change_stream_unwind_transaction_stage.cpp
• src/mongo/db/pipeline/change_stream_filter_helpers.cpp
• src/mongo/db/pipeline/change_stream_rewrite_helpers.cpp

The files make most use of the specific oplog field names used by change streams.