Determine business metrics handling for waiters & paginators

[sichanyoo]

Background

UserAgent v2.1 enhances UserAgent v2.0 by introducing business metrics metadata section.
Out of the list of feature IDs currently supported, we have implementations for waiters and paginators but don't handle setting flags for those currently.

To-do

Figure out how to affect user agent header generation when API call is made through waiter or a paginator.

[sichanyoo]

Write up a design doc for this, even if the design turns out to be relatively simple, we want to avoid hacky approaches being used. The 0.5 time effort is just for the design doc (write-up, discussion, and conclusion).