Add metadata to extension namespaces

The added metadata is used by the Linkerd CLI to recognize the extensions installed in the cluster. Note that this is only required when installing extensions via Helm.

Usage: linkerd-extension-init [OPTIONS] --extension <EXTENSION> --namespace <NAMESPACE> --linkerd-namespace <LINKERD_NAMESPACE>

Options:
      --log-level <LOG_LEVEL>
          [env: LINKERD_NS_LABELER_LOG_LEVEL=]
          [default: info]

      --log-format <LOG_FORMAT>
          [env: LINKERD_NS_LABELER_LOG_FORMAT=]
          [default: plain]

      --extension <EXTENSION>
          Extension name (e.g. viz, multicluster, jaeger)

  -n, --namespace <NAMESPACE>
          Namespace where the extension is installed

      --linkerd-namespace <LINKERD_NAMESPACE>
          Namespace where the Linkerd control-plane is installed

      --prometheus-url <PROMETHEUS_URL>
          URL of external Prometheus instance, if any (only used by the viz extension)

  -h, --help
          Print help (see a summary with '-h')

  -V, --version
          Print version