cntrib/google.golang.org/grpc: fix overwriting outgoing context

[osamingo]

Fixed a bug where the previously set context value will be overwritten.
Also, official docs recommended uses metadata.AppendToOutgoingContext function.

ref. https://github.com/grpc/grpc-go/blob/master/Documentation/grpc-metadata.md#sending-metadata

[dd-caleb]

Thank you for your contribution.

I think the issue may be that we're calling:

md, ok := metadata.FromIncomingContext(ctx)

When we should be calling

md, ok := metadata.FromOutgoingContext(ctx)

Though your solution also probably fixes it. I will take a look later today and add a unit test.

[osamingo]

Thanks for your comment!

I will take a look later today and add a unit test.

OK, I'll be waiting for it.

[osamingo]

I changed to use metadata.FromOutgoingContext function.

[dd-caleb]

Thanks @osamingo. If you wanted to try write the unit test feel free. Otherwise I'll do it in a few hours and merge this PR.

I think its a pretty critical bug since it breaks anything using outgoing metadata, so I'll try to make a release too.

Thanks again.

[dd-caleb]

@osamingo this change has been released as tag v1.2.3. Thanks again for your help!

[osamingo]

@dd-caleb Great, Thank you for a quick response!