Add the /runningpods/ api endpoint (#611)

* Add the /runningpods/ api endpoint

This adds an API endpoint from the kubelet (/runningpods/). It is
an endpoint on kubelet which is considered a "debug" endpoint, so
it might be worth exposing through the options, but by default
it is exposed in most k8s configs AFAICT.

vkubelet/api/pods.go

@@ -0,0 +1,53 @@
+package api
+
+import (
+	"context"
+	"net/http"
+
+	"github.com/cpuguy83/strongerrors"
+	"github.com/virtual-kubelet/virtual-kubelet/log"
+	"k8s.io/api/core/v1"
+	"k8s.io/apimachinery/pkg/runtime"
+	"k8s.io/apimachinery/pkg/runtime/serializer"
+)
+
+// ContainerLogsBackend is used in place of backend implementations for the provider's pods
+type RunningPodsBackend interface {
+	// GetPods retrieves a list of all pods running on the provider (can be cached).
+	GetPods(context.Context) ([]*v1.Pod, error)
+}
+
+func RunningPodsHandlerFunc(p RunningPodsBackend) http.HandlerFunc {
+	scheme := runtime.NewScheme()
+	v1.SchemeBuilder.AddToScheme(scheme)
+	codecs := serializer.NewCodecFactory(scheme)
+
+	return handleError(func(w http.ResponseWriter, req *http.Request) error {
+		ctx := req.Context()
+		ctx = log.WithLogger(ctx, log.L)
+		pods, err := p.GetPods(ctx)
+		if err != nil {
+			return err
+		}
+
+		// Borrowed from github.com/kubernetes/kubernetes/pkg/kubelet/server/server.go
+		// encodePods creates an v1.PodList object from pods and returns the encoded
+		// PodList.
+		podList := new(v1.PodList)
+		for _, pod := range pods {
+			podList.Items = append(podList.Items, *pod)
+		}
+		codec := codecs.LegacyCodec(v1.SchemeGroupVersion)
+		data, err := runtime.Encode(codec, podList)
+		if err != nil {
+			return strongerrors.System(err)
+		}
+
+		w.Header().Set("Content-Type", "application/json")
+		_, err = w.Write(data)
+		if err != nil {
+			return strongerrors.System(err)
+		}
+		return nil
+	})
+}