Removed leaked "database" tag on redis metrics

[PierreF]

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I've found that Redis input plugin may add "database" tag on global redis stats. This is due to tags map being passed by-reference to gatherKeyspaceLine which modify it.

The result is that if you have at least one database in redis (e.g. at least one key), metric result is:

$ ./build/telegraf -config /etc/telegraf/telegraf.conf -config-directory /etc/telegraf/telegraf.d -input-filter redis -test
* Plugin: redis, Collection 1
> redis_keyspace,database=db0,host=xps-pierref,port=6379,role=master,server=172.17.0.5 avg_ttl=0i,expires=0i,keys=50000i 1464869517956355485
> redis_keyspace,database=db1,host=xps-pierref,port=6379,role=master,server=172.17.0.5 avg_ttl=0i,expires=0i,keys=1i 1464869517956396646
> redis,database=db1,host=xps-pierref,port=6379,role=master,server=172.17.0.5 clients=7i,connected_slaves=0i,ev[...] 1464869517956443591

The last line should NOT contains "database=db1" since those metrics are global.
This PR make sure that database tag is not present in redis global metrics.

[sparrc]

This is not a safe operation. The tags map is not copied when it goes elsewhere in the telegraf system. Solution should be to copy the tags map when entering the gatherKeyspaceLine function into a new map.

[PierreF]

It's indeed not copied in Telegraf. As-is is works because it's copied in InfluxDB client code. Agree it's probably not a good idea to rely on currently implementation of client to ensure a copy is done.

I'll do another PR with an explicit copy in gatherKeyspaceLine.

[sparrc]

OK, thanks @PierreF!