Simplify default OBSERVE_REQUEST_CALLBACK behavior

The previous implementation of observe_request(), the default callback
for OBSERVE_REQUEST_CALLBACK,  was checking for
DebugToolbar.is_toolbar_request() and returning True only if that method
returned False.

However, this is actually unneeded, because
DebugToolbarMiddleware never instruments its own requests.  If
DebugToolbar.is_toolbar_request() returns True for a given request,
DebugToolbarMiddleware will return early without performing any
instrumentation or processing on the request [1].

In this case, the OBSERVE_REQUEST_CALLBACK callback never gets called,
because it only gets called via the HistoryPanel.get_headers() method
[2].  The .get_headers() method is only called by DebugToolbarMiddleware
after the early return mentioned above [3].  Thus if
OBSERVE_REQUEST_CALLBACK is called, it must be the case that
DebugToolbar.is_toolbar_request() is False for the current request.
Therefore observe_request() can be simplified to always return True
without changing its behavior.

[1] https://github.com/jazzband/django-debug-toolbar/blob/c688ce4ad7d18c5ecb800869298ca8cf6c08be1d/debug_toolbar/middleware.py#L48-L49
[2] https://github.com/jazzband/django-debug-toolbar/blob/c688ce4ad7d18c5ecb800869298ca8cf6c08be1d/debug_toolbar/panels/history/panel.py#L23-L29
[3] https://github.com/jazzband/django-debug-toolbar/blob/c688ce4ad7d18c5ecb800869298ca8cf6c08be1d/debug_toolbar/middleware.py#L76-L77

Author: living180

debug_toolbar/toolbar.py

@@ -185,4 +185,4 @@ def observe_request(request):
     """
     Determine whether to update the toolbar from a client side request.
     """
-    return not DebugToolbar.is_toolbar_request(request)
+    return True

docs/configuration.rst

@@ -146,9 +146,8 @@ Toolbar options
   Default: ``'debug_toolbar.toolbar.observe_request'``
 
   This is the dotted path to a function used for determining whether the
-  toolbar should update on AJAX requests or not. The default checks are that
-  the request doesn't originate from the toolbar itself, EG that
-  ``is_toolbar_request`` is false for a given request.
+  toolbar should update on AJAX requests or not. The default implementation
+  always returns ``True``.
 
 .. _TOOLBAR_LANGUAGE: