fixed duplicate metric problem at influxDB, on client.WriteSeries

when emitting metrics to influxDB, client.WriteSeries is invoked multiple times.

excerpt from AS-IS
```
2014/05/20 17:51:42 registry: &{map[foo:0xc2100001b8 bar:0xc2100001c0] {0 0}}
data: [{"name":"foo.count","columns":["time","count"],"points":[[1400575902627,58]]}]
data: [{"name":"foo.count","columns":["time","count"],"points":[[1400575902627,58]]},{"name":"bar.value","columns":["time","value"],"points":[[1400575902637,58]]}]
```

expected behavior
```
2014/05/20 17:51:42 registry: &{map[foo:0xc2100001b8 bar:0xc2100001c0] {0 0}}
data: [{"name":"foo.count","columns":["time","count"],"points":[[1400575902627,58]]},{"name":"bar.value","columns":["time","value"],"points":[[1400575902637,58]]}]
```