L13: gRPC Python Client and Server Interceptors

[mehrdada]

Addresses grpc/grpc#8767
Implementation: grpc/grpc#12778
Discussion: https://groups.google.com/d/msg/grpc-io/7qIzyCPWbUA/1NKGXAQHCgAJ

[hsaliak]

@nathanielmanistaatgoogle please approve so that this proposal can be merged in a timely manner.

[mehrdada]

I need to make modifications to it. The API has somewhat changed. Please hold off on reviewing it. I have not forgotten.

[naggie]

After following about 6 links in various PRs and issues around the same change, I'm confused about the status of this interceptor feature -- is it already in 1.8.x? -- I see it in the release notes for v1.8.0

Thanks for the work on this feature, it will allow me to remove considerable boilerplate.

[mehrdada]

@naggie Yes, we pursued this proposal and captured some user feedback and an experimental version (which will very likely become final after marking this proposal as approved) is merged already and is ready to use. I still need to update this proposal to incorporate the feedback and update it to match the version that we shipped since 1.8.

[dfawley]

For consistency, please rename this file to L13-python-... before merging. Thanks!

[lidizheng]

@mehrdada Can you take some time to update the server interceptor section to match the implementation? We want to make interceptor API stable, and it will be great to make this gRFC a reference of its rationale/design.

[mehrdada]

@lidizheng I don't think I am going to have any free time to spend on that, sorry. Feel free to fork it and make it your own; I don't mind. Just to point out one thing you need to figure out that users have asked for is (my memory is a bit fuzzy but it has to be somewhere in the issues) how the interceptors can detect whether an exception thrown is an abortion or not, if I remember correctly.