v0.20.4

Added

• lading.running gauge metric that submits a value of 1 as long as the main
  select! of lading is executing.
• target.running gauge metric that submits a value of 1 as long as the target
  is running.

Changed

• Dogstatsd payload structs now have public fields.
• Capture file descriptor is flushed as soon as current lines are written.
• Renamed Shutdown data type to Phase so that this data type can be used as
  a signal to control lading behavior over distinct stages of load generation
  (e.g., warmup, shutdown).
• Prometheus telemetry and Go expvar target metrics are no longer emitted during
  lading's warmup ("experimental") phase.

Fixed

• Refactors the main select! in lading/src/bin/lading.rs to loop over the
  select to address a potential early-termination bug when the set of generators
  are empty.