Currently, it is not possible to access trailers from within a
stats.Handler. The reason is that both stats.Handler and
ClientStream.Trailer require a lock on the ClientStream.
A workaround would be to start a separate goroutine that will call
ClientStream.Trailer asynchronously, but that requires careful
coordination and we can quite easily make the trailer metadata available
to the stats.Handler directly.
Use case: an interceptor that processes trailer metadata for each
streaming RPC after the stream has finished. Note that a
StreamClientInterceptor returns immediately, before the stream has
finished and before the trailer metadata is available.
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