Context handling in Tracers

In 1.10 and 1.11 the `Context` returned by `ServerStreamTracer.filterContext` was attached when `streamClosed` was invoked. As of 1.12 (and continuing through 1.14) this no longer seems to be the case. It appears this was undocumented behavior and relying on it was my mistake.

That said, I think it _would_ be nice if `ServerStreamTracer` methods were guaranteed to be invoked with the filtered `Context` attached. Either way, it would be helpful to have the Javadoc for `filterContext` specify whether or not the filtered `Context` is attached when other `ServerStreamTracer` methods are invoked.