OnionPerf adds six new key-value pairs to the .tpf format that Torperf/CollecTor did not produce: `ENDPOINTLOCAL`, `ENDPOINTPROXY`, `ENDPOINTREMOTE`, `HOSTNAMELOCAL`, `HOSTNAMEREMOTE`, and `SOURCEADDRESS`. We should add support for these keys to metrics-lib, so that we can start using their values.