Improve GenerationStats to Handle Summation of embed_stats and generation_stats in RAGConvert.convert()

[mdr223]

Summing the embed_stats and generation_stats leads to correct calculations of operator-record, operator, plan, and execution stats.

However, if one were to analyze our stats in post-processing -- they would have no way of parsing which e.g. total_input_tokens came from the embedding step and which came from the generation step.