fix(wsgi): ensure traced iterable does not implement __len__ (#5858)

Streaming responses with [waitress](https://github.com/Pylons/waitress)
is not supported by the wsgi middleware. This incompatibility was
introduced by the following commit:
7ce49bd.

## Reproduction

1. `pip install "waitress>=2.1.2" "ddtrace==1.13.0"
"Flask>=2.2.4,<2.3.0"`
2. Create a flask application and add an endpoint that streams a
response:
   app.py:
    ```
    from waitress import serve
    from flask import Flask
    
    app = Flask(__name__)
    
    @app.route('/large.csv')
    def generate_large_csv():
        def generate():
            for row in [1, 2, 3, 4]:
                yield f"{row}\n"
        return app.response_class(generate(), mimetype='text/csv')
    
    @app.route("/")
    def hello():
        return "Hello, World!"

    serve(app, listen='*:8080')
    ```
3. Run ` python path/to/flask_app/app.py`
3. Run `curl http://localhost:8080/large.csv`
4. Error:
    ```
    ERROR:waitress:Exception while serving /large.csv
    Traceback (most recent call last):
File
"/s1/remote-venvs/tesla/lib/python3.10/site-packages/waitress/channel.py",
line 428, in service
        task.service()
File
"/s1/remote-venvs/tesla/lib/python3.10/site-packages/waitress/task.py",
line 168, in service
        self.execute()
File
"/s1/remote-venvs/tesla/lib/python3.10/site-packages/waitress/task.py",
line 466, in execute
        app_iter_len = len(app_iter)
    TypeError: object of type 'ClosingIterator' has no len()
    ```

## Checklist

- [x] Change(s) are motivated and described in the PR description.
- [x] Testing strategy is described if automated tests are not included
in the PR.
- [x] Risk is outlined (performance impact, potential for breakage,
maintainability, etc).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] [Library release note
guidelines](https://ddtrace.readthedocs.io/en/stable/contributing.html#Release-Note-Guidelines)
are followed.
- [x] Documentation is included (in-code, generated user docs, [public
corp docs](https://github.com/DataDog/documentation/)).
- [x] OPTIONAL: PR description includes explicit acknowledgement of the
performance implications of the change as reported in the benchmarks PR
comment.

## Reviewer Checklist

- [x] Title is accurate.
- [x] No unnecessary changes are introduced.
- [x] Description motivates each change.
- [x] Avoids breaking
[API](https://ddtrace.readthedocs.io/en/stable/versioning.html#interfaces)
changes unless absolutely necessary.
- [x] Testing strategy adequately addresses listed risk(s).
- [x] Change is maintainable (easy to change, telemetry, documentation).
- [x] Release note makes sense to a user of the library.
- [x] Reviewer has explicitly acknowledged and discussed the performance
implications of this PR as reported in the benchmarks PR comment.

---------

Co-authored-by: Yun Kim <[email protected]>

Author: mabdinur

ddtrace/contrib/wsgi/wsgi.py

@@ -88,6 +88,14 @@ def _finish_spans(self):
             self._self_parent_span.finish()
             self._self_span_finished = True
 
+    def __getattribute__(self, name):
+        if name == "__len__":
+            # __len__ is defined by the parent class, wrapt.ObjectProxy.
+            # However this attribute should not be defined for iterables.
+            # By definition, iterables should not support len(...).
+            raise AttributeError("__len__ is not supported")
+        return super(_TracedIterable, self).__getattribute__(name)
+
 
 class _DDWSGIMiddlewareBase(object):
     """Base WSGI middleware class.

releasenotes/notes/fix-wsgi-iterable-len-14777de93b723b00.yaml

@@ -0,0 +1,4 @@
+---
+fixes:
+  - |
+    wsgi: Resolves an issue where accessing the ``__len__`` attribute on traced wsgi middlewares raised a TypeError

tests/contrib/wsgi/test_wsgi.py

@@ -308,3 +308,24 @@ def test_distributed_tracing_nested():
     assert config.wsgi.distributed_tracing is True
     assert resp.status == "200 OK"
     assert resp.status_int == 200
+
+
+def test_wsgi_traced_iterable(tracer, test_spans):
+    # Regression test to ensure wsgi iterable does not define an __len__ attribute
+    middleware = wsgi.DDWSGIMiddleware(application)
+    environ = {
+        "PATH_INFO": "/chunked",
+        "wsgi.url_scheme": "http",
+        "SERVER_NAME": "localhost",
+        "SERVER_PORT": "80",
+        "REQUEST_METHOD": "GET",
+    }
+
+    def start_response(status, headers, exc_info=None):
+        pass
+
+    resp = middleware(environ, start_response)
+    assert hasattr(resp, "__iter__")
+    assert hasattr(resp, "close")
+    assert hasattr(resp, "next") or hasattr(resp, "__next__")
+    assert not hasattr(resp, "__len__"), "Iterables should not define __len__ attribute"