[FlightRPC][Java] CallStatus.metadata should get copied into the gRPC StatusRuntimeException

Reported on the mailing list.
```

I had been doing it that way, but I found that If I was sending something
along the lines of:

ErrorFlightMetadata errorFlightMetadata = new ErrorFlightMetadata();
errorFlightMetadata.insert("my_custom_key", "hello");
listener.error(CallStatus.INTERNAL
.withDescription("Testing description")
.withCause(new RuntimeException("My cause"))
.withMetadata(errorFlightMetadata)
.toRuntimeException());

I would only receive the description back by the time it was throwing a
Python error:

FlightInternalError: gRPC returned internal error, with message:
Testing description. Client context: IOError: Server never sent a data
message. Detail: Internal. gRPC client debug context:
{"created":"@1597858462.086707300","description":"Error received from
peer ipv6:[::1]:12233","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"Testing
description","grpc_status":13}

(catching the exception and checking FlightInternalError.extra_info returns
nothing as well)

However if I manually create a grpc status exception like so:

private static Metadata.Key arrowStatusDetail =
Metadata.Key.of("x-arrow-status-detail-bin",
Metadata.BINARY_BYTE_MARSHALLER);
private static Metadata.Key grpcStatusDetail =
Metadata.Key.of("grpc-status-details-bin", Metadata.BINARY_BYTE_MARSHALLER);
private static Metadata.Key statusCode =
Metadata.Key.of("x-arrow-status", Metadata.ASCII_STRING_MARSHALLER);
private static Metadata.Key message =
Metadata.Key.of("x-arrow-status-message-bin",
Metadata.BINARY_BYTE_MARSHALLER);

Metadata metadata = new Metadata();
metadata.put(arrowStatusDetail, "my_internal_details".getBytes());
metadata.put(grpcStatusDetail, "this_is_in_extra_info".getBytes());
metadata.put(statusCode, "1");
metadata.put(message, "my_internal_message".getBytes());
return new StatusRuntimeException(Status.INTERNAL, metadata);
StatusUtils.toGrpcStatus doesn't copy the metadata from the CallStatus into the gRPC status it constructs.
Then I receive this back on the Python side:

FlightInternalError: my_internal_message. Detail: my_internal_details.
Client context: IOError: Server never sent a data message. Detail:
Internal. gRPC client debug context:
{"created":"@1597859023.244971700","description":"Error received from
peer ipv6:[::1]:12233","file":"src/core/lib/surface/call.cc","file_line":1056,"grpc_message":"","grpc_status":13}

and FlightInternalError.extra_info contains the bytes for
"this_is_in_extra_info" - I can pretty much put whatever I need in there
to add richer metadata and utilize it on the client.

It feels a bit awkward / maybe incorrect to dive into the C++ code to
hijack those metadata keys just to transport extra metadata from Java ->
C++. If the above approach with CallStatus is incorrect for bringing extra
data to the client then let me know.

```
StatusUtils.toGrpcStatus doesn't copy the metadata from the CallStatus into the gRPC status it constructs; I think that's the issue.

**Reporter**: [David Li](https://issues.apache.org/jira/browse/ARROW-9802) / @lidavidm

_{**Note**: *This issue was originally created as [ARROW-9802](https://issues.apache.org/jira/browse/ARROW-9802). Please see the [migration documentation](https://github.com/apache/arrow/issues/14542) for further details.*}

Beginne mit StatusUtils.toGrpcStatus und vergleiche, wie die Methode die gRPC StatusRuntimeException mit den an CallStatus übergebenen Metadaten erstellt. Überprüfe, dass die resultierende Exception diese Metadaten beibehält und dass die auf Clientseite gemeldeten Details verfügbar sind; füge gezielte Tests hinzu oder führe sie aus, falls der umgebende Code solche Tests bereitstellt.

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