[13.x] Default PendingRequest@pool() to use 2 for concurrency

[cosmastech]

The fact that this defaults to null feels like a major footgun. Developers will think that they are getting the benefit of concurrent requests when in fact they are just being executed in series. Within the underlying Guzzle library, passing null does mean to execute without a cap.

Is 2 the right number? I have no idea. But I think anything is better than just defaulting to serial execution. Or we could change it to pass 0 which I think would allow for unbounded concurrency.

[cosmastech]

I modified this test to match how we are testing other pooled exceptions in this file. The exception was being thrown because the pool was running in series rather than actually as a pool. This behavior better lines up with how it should be used (in my opinion).

[taylorotwell]

I'm not sure I'm seeing them execute serially on my end.

For instance, on my machine this does not take 5 seconds:

<?php

use Illuminate\Http\Client\Pool;
use Illuminate\Support\Benchmark;
use Illuminate\Support\Facades\Http;
use Illuminate\Support\Facades\Route;

Route::get('/', function () {
    $time = Benchmark::measure(function () {
        $responses = Http::pool(fn (Pool $pool) => [
            $pool->get('http://laravel.test/test'),
            $pool->get('http://laravel.test/test'),
            $pool->get('http://laravel.test/test'),
            $pool->get('http://laravel.test/test'),
            $pool->get('http://laravel.test/test'),
        ]);

        dump(collect($responses)->map(fn ($response) => (string) $response));
    });

    dd($time);
});

Route::get('/test', function () {
    sleep(1);

    return (string) random_int(1, 100);
});