Commenter @Denis is correct, comma separated labels is the way to specify multiple label key/value pairs as described in the Kubernetes documentation ^[0]. Multiple values are ANDed.

- job_name: 'foo'
  kubernetes_sd_configs:
    - role: pod
      selectors:
        - role: pod
          label: "app=MyApp,type=client"

[0] = https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#label-selectors