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/trace/trace_test.go

@@ -0,0 +1,33 @@
+package trace
+
+import (
+	"testing"
+
+	"github.com/mholt/caddy"
+)
+
+// CreateTestTrace creates a trace middleware to be used in tests
+func CreateTestTrace(config string) (*caddy.Controller, *trace, error) {
+	c := caddy.NewTestController("dns", config)
+	m, err := traceParse(c)
+	return c, m, err
+}
+
+func TestTrace(t *testing.T) {
+	_, m, err := CreateTestTrace(`trace`)
+	if err != nil {
+		t.Errorf("Error parsing test input: %s", err)
+		return
+	}
+	if m.Name() != "trace" {
+		t.Errorf("Wrong name from GetName: %s", m.Name())
+	}
+	err = m.OnStartup()
+	if err != nil {
+		t.Errorf("Error starting tracing middleware: %s", err)
+		return
+	}
+	if m.Tracer() == nil {
+		t.Errorf("Error, no tracer created")
+	}
+}