Build gRPC buffer/request directly

[mheffner]

The gRPC approach to exporting data, eg. via the TraceServiceClient::Export(), makes it difficult to reclaim the memory from the exported payload. You can't use stack allocated structures or build a free-list for the payload buffer because you don't have control of the ExportTraceServiceRequest. At volume this leads to a lot of memory churn.

Instead, use a regular http2 client and build the request buffer directly by serializing the gRPC encoded payload. Build a regular http request with the path set to the gRPC RPC call. This would allow more control of buffer management at scale.