Description
RateLimiter uses usually full seconds:

• time(),
• sleep(),
• \DateTimeImmutable::createFromFormat('U', [time]).

Sometimes however, it goes with microseconds:

• microtime(true),
• usleep()

and could use them even more widely:

• \DateTimeImmutable::createFromFormat('U.u', [microtime]),
• return (float) $interval->format('%f') / 1e6 // microseconds...,
• public function getExpirationTime(): ?float;.

Using float gives an option to set interval in milliseconds:

var_dump(\DateInterval::createFromDateString('200ms')->format('%f'));
// string(6) "200000"

and also to shorten wait time (e.g. 1.8s instead of 2.0s).

Example
SlidingWindow uses time() which means that after running ->wait() it is still considered as not expired.

<?php

use Symfony\Component\RateLimiter\Policy\SlidingWindow;

require_once './vendor/autoload.php';

$window = new SlidingWindow('some_id', 2);
sleep(2); // mimic $limit->wait();
var_dump($window->isExpired()); // false - same second

$window = new class ('some_id', 2) {
    public function __construct(string $id, int $intervalInSeconds)
    {
        $this->windowEndAt = microtime(true) + $intervalInSeconds;
    }

    public function isExpired(): bool
    {
        return microtime(true) > $this->windowEndAt;
    }
};
sleep(2); // mimic $limit->wait();
var_dump($window->isExpired()); // true

Summary
Before I go with a PR I would like to know if this change/improvement sounds like something you might merge in (i.e. using integers for time is not set in stone). It works for me locally.