public static async Task<double> CountFraction<T>(this ChannelReader<T> reader,
                                                       Func<T, bool> condition,
                                                       ChannelParallelOptions options)

using Amphisbaena;
using Amphisbaena.Linq;

...

int[] data = new int[] {1, 2, 3, 4, 5, 6, 7, 8, 9};

double fraction = await data
  .ToChannelReader()
  .CountFraction(item => item < 6);