Drop string metrics

Based on https://github.com/rackerlabs/blueflood/pull/394

Drop String Metrics if so configured
 Also, added support to keep string metrics for certain tenants
Added tests to test this situation, where string metrics are configured to be dropped, but tenant id is excluded
I also highly recommend making all configuration settings fields on the class. static should do fine, but the important bit is to not re-evaluate this for every metric we ingest.