Grpc-web with metadata support

[mineme0110]

No description provided.

[thesamet]

Can we generate both traits with metadata and without metadata traits (and omit the flag that chooses). This enables users to gradually migrate one interface at a time, rather than all at once. The traits should have two names: MyService and MyServiceWithMetadata.

[AlexITC]

@thesamet I wonder whether using a default argument could be enough to keep a single interface, so that existing code is not broken and you are free to use the metadata when you need it, while this breaks binary compatibility (from what I understand), I believe we don't need to worry about it this library as it's marked as experimental.

[thesamet]

OK - let's do a default argument. I want to also another change to align with future intentions with scalapb grpc and present state in zio-grpc, the trait should be:

class MyService[A](f: A=>Metadata, defaultContext: =>A) {
  def unaryMethod(request: RequestType, Context: A=f(defaultContext))
}

We can have an helper that instantiates a client when A = Metadata and f is the identity function, but it helps user to deal with domain object up to the client call.

[AlexITC]

@thesamet the last suggested change seems to require a bit more work, would you mind if we do that in another PR? we would like to continue on the integration which depends on this PR.

[thesamet]

I think that the additional work is pretty marginal - would be preferred over a quick followup with a breaking change.

[thesamet]

Is this ready for review or more changes planned?

[AlexITC]

@thesamet it's ready, your requested changes were applied.

Thanks.

[thesamet]

Merged via 52f7fd4