fix withspan handler nullable + example (#1377)

- class is nullable for pre hooks
- add phpt tests for withspan and its interaction with auto root span

Instrumentation/WithSpanHandler.php

@@ -18,7 +18,7 @@ class WithSpanHandler
     /**
      * @psalm-suppress ArgumentTypeCoercion
      */
-    public static function pre(mixed $target, array $params, string $class, string $function, ?string $filename, ?int $lineno, ?array $span_args = [], ?array $attributes = []): void
+    public static function pre(mixed $target, array $params, ?string $class, string $function, ?string $filename, ?int $lineno, ?array $span_args = [], ?array $attributes = []): void
     {
         static $instrumentation;
         $instrumentation ??= new CachedInstrumentation(name: 'io.opentelemetry.php.with-span', schemaUrl: 'https://opentelemetry.io/schemas/1.25.0');