Progress, Cancellation, Ping, Pagination, and _meta

The primitives are done, the four the client calls and the three the server calls back. What is left is the plumbing they all share. Five cross-cutting utilities run underneath every method: progress reporting, cancellation, ping, pagination, and the reserved _meta field that carries some of them. None is a primitive on its own, and all five show up in real traffic, so this post reads each on the…