Extend statd plugin to support service identifier as lua script

[piotrwielgolaski-tomtom]

I'm using https://docs.konghq.com/kubernetes-ingress-controller/ and
I would like to use in metrics name of service from tag in my case k8s-name

In tcp-log plugin I use

  - name: tcp-log-global
    plugin: tcp-log
    config:
      custom_fields_by_lua:
        service.name: |
          local tags = ngx.ctx.service and ngx.ctx.service.tags
          if tags then
              for _, tag in ipairs(tags) do
              local namespace = string.match(tag, "k8s%-name:(.+)")
              if namespace then
                  return namespace
              end
              end
          end
          return nil

it would be nice to have more freedom to customize service and consumer id in metrics.

[ADD-SP]

@Achiel It seems a new feature, what do you think?