Adds tp_stat conditional around thread_pool metrics collection.

The option to not track `thread_pool` metrics exists however it is not applied in the code (see https://github.com/cotap/sensu-community-plugins/blob/7720498ef1dc952a0a8680c180453dab18660bf2/plugins/elasticsearch/es-node-graphite.rb#L85) . Adding this if/else block around the `thread_pool` metrics collection allows a user to not track `thread_pool` metrics.