Add a helper method observeExecution

Hey!

What do you think about adding a helper method `observeExecution` to `Histogram` class:

```php
public function observeExecution(callable $fn, array $labels = []): void
{
    $start = microtime(true);
    $fn();
    $duration = microtime(true) - $start;
    $this->observe($duration, $labels);
}
```

Can make a PR.