Here's an example of using the REST API to query node metrics and return a []byte in JSON format. Replace "nodes" with "pods" to get pod/container metrics.

data, err := clientset.RESTClient().Get().AbsPath("apis/metrics.k8s.io/v1beta1/nodes").DoRaw()

As explained in the question, the documentations are not clear for a beginner. Even go-client examples retrieve the data, I wanted to get Type support.

As it explained by above answer, you can get the data in []byte in JSON format. This is how I did it.

package main

import (
    "encoding/json"
    "fmt"
    "time"

    "k8s.io/client-go/kubernetes"
    "k8s.io/client-go/rest"
)

// PodMetricsList : PodMetricsList
type PodMetricsList struct {
    Kind       string `json:"kind"`
    APIVersion string `json:"apiVersion"`
    Metadata   struct {
        SelfLink string `json:"selfLink"`
    } `json:"metadata"`
    Items []struct {
        Metadata struct {
            Name              string    `json:"name"`
            Namespace         string    `json:"namespace"`
            SelfLink          string    `json:"selfLink"`
            CreationTimestamp time.Time `json:"creationTimestamp"`
        } `json:"metadata"`
        Timestamp  time.Time `json:"timestamp"`
        Window     string    `json:"window"`
        Containers []struct {
            Name  string `json:"name"`
            Usage struct {
                CPU    string `json:"cpu"`
                Memory string `json:"memory"`
            } `json:"usage"`
        } `json:"containers"`
    } `json:"items"`
}

func getMetrics(clientset *kubernetes.Clientset, pods *PodMetricsList) error {
    data, err := clientset.RESTClient().Get().AbsPath("apis/metrics.k8s.io/v1beta1/pods").DoRaw()
    if err != nil {
        return err
    }
    err = json.Unmarshal(data, &pods)
    return err
}

func main() {
    // creates the in-cluster config
    // https://github.com/kubernetes/client-go/tree/master/examples#configuration
    config, err := rest.InClusterConfig()
    if err != nil {
        panic(err.Error())
    }
    // creates the clientset
    clientset, err := kubernetes.NewForConfig(config)
    if err != nil {
        panic(err.Error())
    }
    var pods PodMetricsList
    err = getMetrics(clientset, &pods)
    if err != nil {
        panic(err.Error())
    }
    for _, m := range pods.Items {
        fmt.Println(m.Metadata.Name, m.Metadata.Namespace, m.Timestamp.String())
    }
}

Install following Go packages: go get -u k8s.io/client-go/kubernetes k8s.io/client-go/rest

You can use following endpoints to retrieve the data as you want;

• Nodes: apis/metrics.k8s.io/v1beta1/nodes
• Pods: apis/metrics.k8s.io/v1beta1/pods
• Pods of default namespace: apis/metrics.k8s.io/v1beta1/namespaces/default/pods
• Specific Pod: /apis/metrics.k8s.io/v1beta1/namespaces/default/pods/<POD-NAME>

NOTE: You may need to change the Type before json.Unmarshal. You can define the Type only for the field which you are interested with.

There's a much better API for this: https://github.com/kubernetes/metrics. Using this, you don't have to create the data structs or handle row byte slices.

import (
  metav1 "k8s.io/apimachinery/pkg/apis/meta/v1"
  metricsv "k8s.io/metrics/pkg/client/clientset/versioned"
  ...
)

...
clientset, err := metricsv.NewForConfig(config)
podMetricsList, err := clientset.MetricsV1beta1().PodMetricses("").List(metav1.ListOptions{})