Duplicated metrics for restarted Pods

I believe I have encountered a bug where multiple values are exported for the same Pod at the same point in time when that Pod has been restarted.

I have been doing some load tests against an app in K8s and I noticed something. The Pod had a limit set to 1Gi and while I was attacking the app with requests the Pods restarted a few times.

When I looked at the graphs in Grafana, it seemed like Pods are using way over 2.6GiB of memory. That didn't make much sense so I investigated the query, which led me to finding this issue.

Querying the `container_memory_working_set_bytes` metric in Prometheus I got the following:

```ini
container_memory_working_set_bytes{namespace="my-app", container="my-app"}
# result:
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/pod80ea171a-411f-48cf-a603-04bd71a03c1e/372804cd3c321ca3f908265296407d3e9f6bd06568aab5f7be7f7047081dd7dc", image="my-image", instance="172.22.218.162:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-zc7ll_my-app_80ea171a-411f-48cf-a603-04bd71a03c1e_7", namespace="my-app", node="node1", pod="web-5dfd4896b4-zc7ll"} 1038004224
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/pod80ea171a-411f-48cf-a603-04bd71a03c1e/8d37738784860ae70513378dba0df77acc15e52001cae5d73ab6799533d06a4d", image="my-image", instance="172.22.218.162:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-zc7ll_my-app_80ea171a-411f-48cf-a603-04bd71a03c1e_8", namespace="my-app", node="node1", pod="web-5dfd4896b4-zc7ll"} 815841280
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/pod80ea171a-411f-48cf-a603-04bd71a03c1e/89d3021410ad8d22f36b53ac714ad962c34a5debb4f6de60c7056620d0721156", image="my-image", instance="172.22.218.162:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-zc7ll_my-app_80ea171a-411f-48cf-a603-04bd71a03c1e_9", namespace="my-app", node="node1", pod="web-5dfd4896b4-zc7ll"} 948023296
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/pod80ea171a-411f-48cf-a603-04bd71a03c1e/4300c0a162a494e32ab02c5b3741d91cc6f4b22cc9049a8cdc3d008b57e2dd8b", image="my-image", instance="172.22.218.162:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-zc7ll_my-app_80ea171a-411f-48cf-a603-04bd71a03c1e_10", namespace="my-app", node="node1", pod="web-5dfd4896b4-zc7ll"} 85966848
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/podb7ecf824-6797-4183-9566-434142df3757/e384a937574526eebeee4438132a0e882c0517bce4ca84db14497d598cd93a09", image="my-image", instance="172.22.106.15:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-x4nsr_my-app_b7ecf824-6797-4183-9566-434142df3757_6", namespace="my-app", node="node2", pod="web-5dfd4896b4-x4nsr"} 980393984
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/podb7ecf824-6797-4183-9566-434142df3757/e5c6aec2cc6cf7abbd2163cd195b181954df28fc134478e2cfd27283c2a7838f", image="my-image", instance="172.22.106.15:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-x4nsr_my-app_b7ecf824-6797-4183-9566-434142df3757_7", namespace="my-app", node="node2", pod="web-5dfd4896b4-x4nsr"} 915144704
container_memory_working_set_bytes{container="my-app", endpoint="https-metrics", id="/kubepods/burstable/podb7ecf824-6797-4183-9566-434142df3757/fcce62cda4d8300c82f4552ddd069b59e8de30c31ece187a6349fe786f182e7a", image="my-image", instance="172.22.106.15:10250", job="kubelet", metrics_path="/metrics/cadvisor", name="k8s_my-app_web-5dfd4896b4-x4nsr_my-app_b7ecf824-6797-4183-9566-434142df3757_8", namespace="my-app", node="node2", pod="web-5dfd4896b4-x4nsr"} 799879168
```

Notice how individual starts of the Pods are recorded for the same point in time - see the `name` labels ending `_7`, `_8`, `_9`, `_10` for example. These are four instances of the same exact Pod but in reality they never ran at the same time (they're restarts). If I add together these values it will give 2.6GiB which is the number I saw in Grafana. I can confirm this from other graphs that memory usage on the nodes never registered a 2.6GiB increase but they saw 1GiB, which is my limit.

I use Amazon EKS with Kubernetes version v1.19.6-eks-49a6c0 which I believe uses cAdvisor [v0.37.3](https://github.com/kubernetes/kubernetes/blob/v1.19.7/CHANGELOG/CHANGELOG-1.19.md#dependencies).