docs: Add a DEBUG.md

[bshaffer]

See https://github.com/googleapis/google-cloud-go/blob/main/debug.md

Similary, PHP could have one. Here's an example of how to add debugging to bigquery

use Google\Cloud\BigQuery\BigQueryClient;
use GuzzleHttp\HandlerStack;
use Google\Auth\HttpHandler\HttpHandlerFactory;
use GuzzleHttp\Middleware;
use GuzzleHttp\MessageFormatter;
use Monolog\Logger;
use Monolog\Level;
use Monolog\Handler\StreamHandler;

$logger = new Logger('Logger');
$logger->pushHandler(new StreamHandler('php://stdout', Level::Debug)); // <<< uses a stream

$stack = HandlerStack::create();
$stack->push(
    Middleware::log(
        $logger,
        new MessageFormatter('{req_body} - {res_body}')
    )
);
$client = new \GuzzleHttp\Client([
    'handler' => $stack,
]);
$httpHandler = HttpHandlerFactory::build($client);

$bigQuery = new BigQueryClient([
    'httpHandler' => $httpHandler,
    'projectId' => $projectId,
]);
$datasets = $bigQuery->datasets();
foreach ($datasets as $dataset) {
    print($dataset->id() . PHP_EOL);
}

This would be similar for any GAPIC client as well, although you'd need to supply httpHandler to the transport config, and it would only work for REST requests

use Google\Cloud\Dlp\V2\Client\DlpServiceClient;
use Google\Cloud\Dlp\V2\ListInfoTypesRequest;
use GuzzleHttp\HandlerStack;
use Google\Auth\HttpHandler\HttpHandlerFactory;
use GuzzleHttp\Middleware;
use GuzzleHttp\MessageFormatter;
use Monolog\Logger;
use Monolog\Level;
use Monolog\Handler\StreamHandler;

$logger = new Logger('Logger');
$logger->pushHandler(new StreamHandler('php://stdout', Level::Debug)); // <<< uses a stream

$stack = HandlerStack::create();
$stack->push(
    Middleware::log(
        $logger,
        new MessageFormatter('{req_body} - {res_body}')
    )
);
$client = new \GuzzleHttp\Client([
    'handler' => $stack,
]);
$httpHandler = [HttpHandlerFactory::build($client), 'async'];

// Instantiate a client.
$dlp = new DlpServiceClient([
    'transportConfig' => ['rest' => ['httpHandler' => $httpHandler]],
]);

// Run request
$listInfoTypesRequest = (new ListInfoTypesRequest())
    ->setLanguageCode($languageCode)
    ->setFilter($filter);
$response = $dlp->listInfoTypes($listInfoTypesRequest);

// Print the results
print('Info Types:' . PHP_EOL);
foreach ($response->getInfoTypes() as $infoType) {
    printf(
        '  %s (%s)' . PHP_EOL,
        $infoType->getDisplayName(),
        $infoType->getName()
    );
}