I think there are two reasonable approaches - 1) Iterator style. In this case we either create a new iterator type or re-use the FieldHandle as an iterator. In this case you get a handle to the first field, and then re-use it for each field. This saves the expense of allocating a new handle for every field.
2) Functor style. In this case we do the equivalent of std::find_if(). It would be similar to TSRecordDump, the differences being it is over the HTTP header fields, and the functor can return a "stop calling me" value to terminate the iteration. In terms of implementation, the latter should be noticeably easier. While I think option (1) is more flexible, I favor option (2) for ease of implementation because it's mainly clunkier than (1), not really much less powerful. Having looked at the underlying data structures, making option (1) efficient would involve some non-trivial changes to structures I am not sure anybody understands. I think option (2) could be done in a day, on the other hand.
