Note that `push_to_gateway`/`pushadd_to_gateway` now requires an explicit registry argument, as the usual use case for batch jobs is to push a small custom registry.
[FEATURE] Add `time()` to gauges
[FEATURE] Provide MetricFamilies to make custom collectors easier
[FEATURE] Graphite bridge now allows adding a namespace prefix to exported sample names
[FEATURE] Text format parser
[IMPROVEMENT] Improved docstrings
[IMPROVEMENT] `push_to_gateway`/`pushadd_to_gateway` now requires explicit registry argument
[ENHANCEMENT] Allow value implementation to be pluggable
[ENHANCEMENT] Removed resource warning in tests
[BUGFIX] Pushgateway tests now work on Python 3.4+