Move around some packages (#658)

* Move tracing exporter registration

This doesn't belong in the library and should be configured by the
consumer of the opencensus package.

* Rename `vkublet` package to `node`

`vkubelet` does not convey any information to the consumers of the
package.
Really it would be nice to move this package to the root of the repo,
but then you wind up with... interesting... import semantics due to the
repo name... and after thinking about it some, a subpackage is really
not so bad as long as it has a name that convey's some information.

`node` was chosen since this package deals with all the semantics of
operating a node in Kubernetes.

node/api/stats.go

@@ -0,0 +1,55 @@
+package api
+
+import (
+	"context"
+	"encoding/json"
+	"net/http"
+
+	"github.com/pkg/errors"
+	stats "k8s.io/kubernetes/pkg/kubelet/apis/stats/v1alpha1"
+)
+
+// PodStatsSummaryHandlerFunc defines the handler for getting pod stats summaries
+type PodStatsSummaryHandlerFunc func(context.Context) (*stats.Summary, error)
+
+// HandlePodStatsSummary makes an HTTP handler for implementing the kubelet summary stats endpoint
+func HandlePodStatsSummary(h PodStatsSummaryHandlerFunc) http.HandlerFunc {
+	if h == nil {
+		return NotImplemented
+	}
+	return handleError(func(w http.ResponseWriter, req *http.Request) error {
+		stats, err := h(req.Context())
+		if err != nil {
+			if isCancelled(err) {
+				return err
+			}
+			return errors.Wrap(err, "error getting status from provider")
+		}
+
+		b, err := json.Marshal(stats)
+		if err != nil {
+			return errors.Wrap(err, "error marshalling stats")
+		}
+
+		if _, err := w.Write(b); err != nil {
+			return errors.Wrap(err, "could not write to client")
+		}
+		return nil
+	})
+}
+
+func isCancelled(err error) bool {
+	if err == context.Canceled {
+		return true
+	}
+
+	if e, ok := err.(causal); ok {
+		return isCancelled(e.Cause())
+	}
+	return false
+}
+
+type causal interface {
+	Cause() error
+	error
+}