Feature/better metrics

[xiejiangzhi]

Support multiple metrics
Support custom metrics

[xiejiangzhi]

<match statsd>
  type statsd
  host localhost # optional
  port 8125 # optional
  namespace a.b.c # optional

  <metric>
    statsd_type timing
    statsd_key my_app.nginx.response_time
    statsd_val ${record['response_time']}
  </metric>

  <metric>
    statsd_type incrument
    statsd_key my_app.nginx.${response_code.to_i / 100}xx # 2xx 4xx 5xx
  </metric>

  # more <metric>...</metric>
</match>