Ensure that, in multi-turn conversations, we include the full conversation history, not just the last message

Currently, we "pop" the last message in a conversation and only add that as the `gen_ai.request.messages` value in order to manage the size of the span (we don't want to accidentally exceed it).

The ability to include the full conversation history is blocked by the "span first" feature. Once it's released, we need to revisit this to ensure that we're retaining the entire conversation history passed in `contents` in `generate_content` calls on the `gen_ai.request.messages` attribute on spans.