Allow server implementation to access extra attributes

Current method signature: (t:T, ctx: Headers) lacks ability to get attributes from outside.

Imagine when you implement an auth interceptor that put a user id to request's attributes, how can you get the user id in service implementation?

val routes = TestService.toRoutes(impl)
val withAuth = HttpRoutes[IO] { req =>
    OptionT.liftF(decodeJWT(req.headers.get(ci"Authorization").head)).flatMap {
      claims => routes.run(req.withAttributes(userIdKey, claims.sub))
    }
}

I have two (similar) idea to implement this feature.

The first idea, which draws inspiration from Rust's tonic, is to define gRPC Request type to carry extra data and generated method signature becomes t: Request[T].

trait Request[T] {
  def message: T
  def headers: Headers // For accuracy, we should us e grpc MetadataMap type instead of http.Headers
  def attributes: Vault
}

// ...
  def unaryToUnary[F[_]: Temporal, A, B](
      decode: Decoder[A],
      encode: Encoder[B],
      serviceName: String,
      methodName: String,
  )( // Stuff we apply at invocation
-     f: (A, Headers) => F[B]
+.    f: (Request[A]) => F[B]
 ): HttpRoutes[F] = HttpRoutes.of[F] {

The second idea is to just add Vault type to f arguments.

// ...
  def unaryToUnary[F[_]: Temporal, A, B](
      decode: Decoder[A],
      encode: Encoder[B],
      serviceName: String,
      methodName: String,
  )( // Stuff we apply at invocation
-     f: (A, Headers) => F[B]
+.    f: (A, Headers, Vault) => F[B]
 ): HttpRoutes[F] = HttpRoutes.of[F] {

http4s / http4s-grpc

Allow server implementation to access extra attributes #144