Added support for tags at reporter level

This enables tags at reporter level. Currently there is no way to associate tags for JVM metrics. Also global tags like environment, name of app, version etc if set at reporter level could help having *not* to use them for each metric construction.
I would rename `tags` here and in Gauge and Series to `globalTags` or `additionalTags` to make it clear these tags are in addition to those embedded in the metric name.