Only update delta in subscribe2

[neekolas]

Summary

In Subscribe2 we allow a client to update their subscription by passing an updated list of the topics they are interested in. The subscription metrics were incrementing the total subs by the length of the list instead of the delta between the old and new, causing our subscriptions metric to inflate.

Notes

One caveat to this approach is that we only set the xmtp_subscribe_topics_length based on the original request, not any updates. That seems better than observing a new metric every time the subscription changes by 1.