Improper modification of global tags when bucketing histogram, meter, timer metrics

[nitti]

When creating tags for the purpose of bucketing histogram, meter, and timer metrics, the library improperly modifies the global tag set:

https://github.com/vrischmann/go-metrics-influxdb/blob/master/influxdb.go#L151
https://github.com/vrischmann/go-metrics-influxdb/blob/master/influxdb.go#L173
https://github.com/vrischmann/go-metrics-influxdb/blob/master/influxdb.go#L207

I'm sure the intent here was to copy the existing tag set, but maps are a reference type. The result is that the bucket tag that is added for histogram, meter, and timer metrics is also applied to counter and gauge metrics.

The correct way to handle this is to explicitly copy the existing map:

these_tags := map[string]string{}
for tk, tv := range r.tags {
    these_tags[tk] = tv
}

I'll open a PR with the fix.