Create a Reader from an iterable of Uint8Arrays.

     import { readerFromIterable } from "https://deno.land/std@0.191.0/streams/reader_from_iterable.ts";
     import { copy } from "https://deno.land/std@0.191.0/streams/copy.ts";

     const file = await Deno.open("metrics.txt", { write: true });
     const reader = readerFromIterable((async function* () {
       while (true) {
         await new Promise((r) => setTimeout(r, 1000));
         const message = `data: ${JSON.stringify(Deno.metrics())}\n\n`;
         yield new TextEncoder().encode(message);
       }
     })());
     await copy(reader, file);