Allow client to specify metadata per call

[cvanderschuere]

Currently the client is only able to modify the metadata at the service level, but metadata is meant to be per-call.

grpc-swift/Sources/protoc-gen-swiftgrpc/Generator-Client.swift

Lines 189 to 194 in dd4d1cb

	println("/// Asynchronous. Unary.")
	println("\(access) func \(methodFunctionName)(_ request: \(methodInputName), completion: @escaping (\(methodOutputName)?, CallResult) -> Void) throws -> \(callName) {")
	indent()
	println("return try \(callName)Base(channel)")
	indent()
	println(".start(request: request, metadata: metadata, completion: completion)")

Changes:

• Allow an optional metadata to be passed per call
• Add merge to Metadata
• Merge service metadata and call metadata on call.start

[MrMage]

Note that we can't really make arguments optional, as that is currently not possible on protocols (and we switched to protocols for better test ability). Although, it might be possible to add protocol extensions with optional arguments; that could be a viable workaround.

Do you need per-call metadata at the moment? I would have supposed it's more of an edge case, with the most common case being per-channel authentication metadata.

[cvanderschuere]

Yes we do. The most direct use case is for request tracing which must be via per-rpc metadata. Non-optional is certainly reasonable from our perspective since we will be using it every time anyways.