Add a 3rd generic parameter to HttpClientTracer for HEADERS

[trask]

HttpClientTracer.startScope() and .getSetter() are not working in some places because the object we want to inject the header into does not have the same type as the request, e.g. #922 (comment)

If there are a few of these places, it may make sense to add a 3rd generic parameter to HttpClientTracer for HEADERS, to make startScope() and getSetter() more broadly applicable.

[trask]

As part of this, see if we can get rid of all getSetters() that return null