Group metrics.

Produces grouped multi-metrics like this:

```
source=Erics-MacBook-Pro.local.127.0.0.1:11211 measure.memcached.pid=306 measure.memcached.uptime=10936 measure.memcached.time=1375904731 measure.memcached.version=1.4.15 measure.memcached.libevent=2.0.21-stable measure.memcached.pointer_size=64 measure.memcached.rusage_user=0.145808 measure.memcached.rusage_system=0.205689 measure.memcached.curr_connections=16 measure.memcached.total_connections=25 measure.memcached.connection_structures=23 measure.memcached.reserved_fds=20 measure.memcached.cmd_get=0 measure.memcached.cmd_set=0 measure.memcached.cmd_flush=0 measure.memcached.cmd_touch=0 measure.memcached.get_hits=0 measure.memcached.get_misses=0 measure.memcached.delete_misses=0 measure.memcached.delete_hits=0 measure.memcached.incr_misses=0 measure.memcached.incr_hits=0 measure.memcached.decr_misses=0 measure.memcached.decr_hits=0 measure.memcached.cas_misses=0 measure.memcached.cas_hits=0 measure.memcached.cas_badval=0 measure.memcached.touch_hits=0 measure.memcached.touch_misses=0 measure.memcached.auth_cmds=0 measure.memcached.auth_errors=0 measure.memcached.bytes_read=480 measure.memcached.bytes_written=16652 measure.memcached.limit_maxbytes=67108864 measure.memcached.accepting_conns=1 measure.memcached.listen_disabled_num=0 measure.memcached.threads=4 measure.memcached.conn_yields=0 measure.memcached.hash_power_level=16 measure.memcached.hash_bytes=524288 measure.memcached.hash_is_expanding=0 measure.memcached.bytes=0 measure.memcached.curr_items=0 measure.memcached.total_items=0 measure.memcached.expired_unfetched=0 measure.memcached.evicted_unfetched=0 measure.memcached.evictions=0 measure.memcached.reclaimed=0
source=Erics-MacBook-Pro.local measure.sidekiq.queues.processed=1410 measure.sidekiq.queues.failed=189 measure.sidekiq.queues.enqueued=0 measure.sidekiq.workers.busy=0
source=Erics-MacBook-Pro.local.emails measure.sidekiq.queue.enqueued=0
source=Erics-MacBook-Pro.local.deliveries measure.sidekiq.queue.enqueued=0
source=Erics-MacBook-Pro.local.background measure.sidekiq.queue.enqueued=0
source=Erics-MacBook-Pro.local.events measure.sidekiq.queue.enqueued=0
source=Erics-MacBook-Pro.local.messages measure.sidekiq.queue.enqueued=0
source=Erics-MacBook-Pro.local.pusher measure.sidekiq.queue.enqueued=0
```

not sure if long log lines length will be a problem or not, but easy enough to fix in formatted-metrics if it is.