-type key() :: iodata().

-type operation() ::
          {cast, iodata()} |
          {counter, key(), value(), sample_rate()} |
          {gauge, key(), value()} |
          {gauge_decrement, key(), value()} |
          {gauge_increment, key(), value()} |
          {timing, key(), value()} |
          {timing_now, key(), erlang:timestamp()} |
          {timing_now_us, key(), erlang:timestamp()}.

-type sample_rate() :: number().

-type value() :: number().