Add Trailer to client-side stats.End

[mklencke]

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.

[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

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Regards,
CLA GitHub bot

[dfawley]

This LGTM but you need to make sure to make the CLA bot happy. Thanks!

[mklencke]

I added my google.com account and the CLA should now be good. Please retry CLA verification.

[dfawley]

@mklencke that doesn't seem to have worked. Is the email address on your commit your @google.com email address?

[thelinuxfoundation]

Thank you for your pull request. Before we can look at your contribution, we need to ensure all contributors are covered by a Contributor License Agreement.

After the following items are addressed, please respond with a new comment here, and the automated system will re-verify.

• User @mklencke isn't covered by a CLA. They will need to complete the form at https://identity.linuxfoundation.org/projects/cncf

Regards,
CLA GitHub bot

[dfawley]

That looks like a better error than the first one. I'm not sure it's required for Google employees to sign the CLA directly, but that is what we did on the gRPC team and it made everything work. So if you can't find some other way to take care of it, it's probably best to go through the process directly (it's pretty quick: click the link above, sign up as an employee, sign in with google using your corp account, link your github account, agree to the CLA, then drop a comment here to wake the bot up).

[mklencke]

bot, please retry :)

[mklencke]

I signed it