The C++ gRPC server sometimes returns both headers and trailers. An
excerpt from Wireshark:
```
Stream: HEADERS, Stream ID: 1, Length 136, 200 OK
Header: :status: 200 OK
Header: x-middleware: expected value
Header: content-type: application/grpc
Header: grpc-accept-encoding: identity,deflate,gzip
Header: accept-encoding: identity,gzip
Stream: HEADERS, Stream ID: 1, Length 92
Header: grpc-status: 2
Header: grpc-message: Unknown
Header: x-arrow-status: 9
Header: x-arrow-status-message-bin: VW5rbm93bg
```
Before this commit, only the metadata from the trailer would be
available, missing the `x-middleware` header:
```
MetadataMap {
headers: {
"x-arrow-status-message-bin": "VW5rbm93bg",
"x-arrow-status": "9",
},
}
```
