add Django REST framework support

README.rst

@@ -54,6 +54,8 @@ If you want to directly override the ``DjangoTracing`` used, you can use the fol
 
     # some_opentracing_tracer can be any valid OpenTracing tracer implementation
     OPENTRACING_TRACING = django_opentracing.DjangoTracing(some_opentracing_tracer)
+    # for Django REST framework users
+    OPENTRACING_TRACING = django_opentracing.DjangoRestFrameworkTracing(some_opentracing_tracer)
 
 **Note:** Valid request attributes to trace are listed [here](https://docs.djangoproject.com/en/1.9/ref/request-response/#django.http.HttpRequest). When you trace an attribute, this means that created spans will have tags with the attribute name and the request's value.
 

django_opentracing/tracing.py

@@ -145,6 +145,18 @@ def _call_start_span_cb(self, span, request):
             pass
 
 
+class DjangoRestFrameworkTracing(DjangoTracing):
+    '''tracer for Django REST framework
+    '''
+    def get_span(self, request):
+        '''
+        @param request
+        Returns the span tracing this request
+        '''
+        scope = self._current_spans.get(request._request, None)
+        return None if scope is None else scope.span
+
+
 def initialize_global_tracer(tracing):
     '''
     Initialisation as per https://github.com/opentracing/opentracing-python/blob/9f9ef02d4ef7863fb26d3534a38ccdccf245494c/opentracing/__init__.py#L36 # noqa