Allow for Director to set OutgoingMetadata on proxied calls

[mwitkow]

As mentioned in
improbable-eng/grpc-web#91
and in discussion of #16 (@peteredge's comment) we need a way to add metadata to outgoing calls.

The improbable-eng/grpc-web#91 broke because of gRPC-Go changes in 1.5 that separated the inbound and output contexts. This meant that it no longer forwarded the metadata.

I propose the following change:

type StreamDirector func(ctx context.Context, fullMethodName string) (*grpc.ClientConn, error)

to

type StreamDirector func(ctx context.Context, fullMethodName string) (context.Context, *grpc.ClientConn, error)

Where the returned context will be used as a base for the call.

Thoughts? @yifanzz

[cleanerx]

Just ran into this problem. The readme needs adjustment as well, since it reflects to old behavior

[yifanzz]

@mwitkow that looks good to me. Will create a PR for the change.

[ghost]

It seems this was fixed in last commit but issue was not closed.