Merge pull request #4 from grepory/master

Don't attempt to relay empty metrics
Merge pull request #5 from grepory/master

Add empty json fixture
If the queue is empty, return 0 for queue length
Merge pull request #6 from grepory/fix_empty_queue

If the queue is empty, return 0 for queue length
Ensure mutator adds tags included in JSON metrics
Merge pull request #7 from grepory/fix_json_tags

Ensure mutator adds tags included in JSON metrics