Grpc tracing (#544)

* checkpoint

* Pass context through ServeDNS, enable gRPC tracing

* Fix types and make tracer available to proxy. go fmt

* Fix imports

* Use the DoNotStartTrace option

* Change to SpanFilter from DoNotStartTrace

* Use new name (IncludeSpan)

* Final names

* Add tests; fix possible client/conn leaks in grpc

* go fmt

middleware/proxy/proxy.go

@@ -28,6 +28,10 @@ type Proxy struct {
 	// midway.
 
 	Upstreams *[]Upstream
+
+	// Trace is the Trace middleware, if it is installed
+	// This is used by the grpc exchanger to trace through the grpc calls
+	Trace middleware.Handler
 }
 
 // Upstream manages a pool of proxy upstream hosts. Select should return a