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/grpc_test.go

@@ -0,0 +1,54 @@
+package proxy
+
+import (
+	"testing"
+	"time"
+)
+
+func pool() []*UpstreamHost {
+	return []*UpstreamHost{
+		{
+			Name: "localhost:10053",
+		},
+		{
+			Name: "localhost:10054",
+		},
+	}
+}
+
+func TestStartupShutdown(t *testing.T) {
+	upstream := &staticUpstream{
+		from:        ".",
+		Hosts:       pool(),
+		Policy:      &Random{},
+		Spray:       nil,
+		FailTimeout: 10 * time.Second,
+		MaxFails:    1,
+	}
+	g := newGrpcClient(nil, upstream)
+	upstream.ex = g
+
+	p := &Proxy{Trace: nil}
+	p.Upstreams = &[]Upstream{upstream}
+
+	err := g.OnStartup(p)
+	if err != nil {
+		t.Errorf("Error starting grpc client exchanger: %s", err)
+		return
+	}
+	if len(g.clients) != len(pool()) {
+		t.Errorf("Expected %d grpc clients but found %d", len(pool()), len(g.clients))
+	}
+
+	err = g.OnShutdown(p)
+	if err != nil {
+		t.Errorf("Error stopping grpc client exchanger: %s", err)
+		return
+	}
+	if len(g.clients) != 0 {
+		t.Errorf("Shutdown didn't remove clients, found %d", len(g.clients))
+	}
+	if len(g.conns) != 0 {
+		t.Errorf("Shutdown didn't remove conns, found %d", len(g.conns))
+	}
+}