try to resolve FIXME share trailers through matval

[laglangyue]

source code

pekko-grpc/runtime/src/main/scala/org/apache/pekko/grpc/internal/PekkoNettyGrpcClientGraphStage.scala

Lines 169 to 176 in 83bdd04

	def onCallClosed(status: Status, trailers: Metadata): Unit = {
	if (status.isOk()) {
	// FIXME share trailers through matval
	completeStage()
	} else {
	failStage(status.asRuntimeException(trailers))
	}
	call = null

I've submit a pr

[laglangyue]

I went to learn about GRPC, header=>message=>trailers
We stored the header and future [trailers] through GrpcResponseMetadata
We have completed the promise [GrpcResponseMetada] in onHeader, so we are unable to assign a value to the trainers,
So the original author defined another Promise [trailers]
When on Hedaer, GrpcResponseMetadata is in a semi completed state
OnClose is the final state

[laglangyue]

I think this FIXME may not be necessary, but there is a lot of repetitive code about GrpcResponseMetada, which can be constructed using a builder pattern to create GrpcResponseMetada, with Header, with Message, with Close (with Trailers)

I have tried writing some code locally, but I still need some deeper exploration

class GrpcResponseMetadataImpl(
    grpcHeaders: io.grpc.Metadata, trailersPromise: Promise[io.grpc.Metadata]) extends GrpcResponseMetadata {

  override def headers: pekko.grpc.scaladsl.Metadata =
    MetadataImpl.scalaMetadataFromGoogleGrpcMetadata(grpcHeaders)
  override def getHeaders(): pekko.grpc.javadsl.Metadata =
    MetadataImpl.javaMetadataFromGoogleGrpcMetadata(grpcHeaders)
  override def trailers: Future[pekko.grpc.scaladsl.Metadata] =
    trailersPromise.future.map(MetadataImpl.scalaMetadataFromGoogleGrpcMetadata)(ExecutionContexts.parasitic)
  override def getTrailers(): CompletionStage[pekko.grpc.javadsl.Metadata] =
    trailersPromise.future.map(MetadataImpl.javaMetadataFromGoogleGrpcMetadata)(ExecutionContexts.parasitic)
      .asJava
}