[Django] Added support for traceresponse headers

Added opt-in support to return traceresponse headers from Django.

This allows users to configure their Django apps to inject trace context
as headers in HTTP responses. This is useful when client side apps need
to connect their spans with the server side spans e.g, in RUM products.

Today the most practical way to do this is to use the `Server-Timing`
header but in near future we might use the `traceresponse` header as
described here: https://w3c.github.io/trace-context/#trace-context-http-response-headers-format

As a result the implementation does not use a hard-coded header and
instead let's the users pick one.

This can be done by setting the `OTEL_PYTHON_TRACE_RESPONSE_HEADER` to
the header name that users want to inject in HTTP responses. The option
does not have a default value and the feature is disbaled when a env var
is not set.

CHANGELOG.md

@@ -21,6 +21,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#299](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/299))
 - `opentelemetry-instrumenation-django` now supports request and response hooks.
   ([#407](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/407))
+- Add trace response header support for Django.
+  ([#395](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/395))
 
 ### Removed
 - Remove `http.status_text` from span attributes

instrumentation/opentelemetry-instrumentation-django/src/opentelemetry/instrumentation/django/middleware.py

@@ -20,6 +20,9 @@
 
 from opentelemetry.context import attach, detach
 from opentelemetry.instrumentation.django.version import __version__
+from opentelemetry.instrumentation.propagators import (
+    get_global_back_propagator,
+)
 from opentelemetry.instrumentation.utils import extract_attributes_from_object
 from opentelemetry.instrumentation.wsgi import (
     add_response_attributes,
@@ -179,6 +182,11 @@ def process_response(self, request, response):
                 response,
             )
 
+            propagator = get_global_back_propagator()
+            if propagator:
+                propagator.inject(response)
+
+            # record any exceptions raised while processing the request
             exception = request.META.pop(self._environ_exception_key, None)
             if _DjangoMiddleware._otel_response_hook:
                 _DjangoMiddleware._otel_response_hook(  # pylint: disable=not-callable

instrumentation/opentelemetry-instrumentation-django/tests/test_middleware.py

@@ -26,10 +26,19 @@
     DjangoInstrumentor,
     _DjangoMiddleware,
 )
+from opentelemetry.instrumentation.propagators import (
+    TraceResponsePropagator,
+    set_global_back_propagator,
+)
 from opentelemetry.sdk.trace import Span
 from opentelemetry.test.test_base import TestBase
 from opentelemetry.test.wsgitestutil import WsgiTestBase
-from opentelemetry.trace import SpanKind, StatusCode
+from opentelemetry.trace import (
+    SpanKind,
+    StatusCode,
+    format_span_id,
+    format_trace_id,
+)
 from opentelemetry.util.http import get_excluded_urls, get_traced_request_attrs
 
 # pylint: disable=import-error
@@ -41,6 +50,7 @@
     route_span_name,
     traced,
     traced_template,
+    with_response_header,
 )
 
 DJANGO_2_2 = VERSION >= (2, 2)
@@ -52,6 +62,7 @@
     url(r"^excluded_arg/", excluded),
     url(r"^excluded_noarg/", excluded_noarg),
     url(r"^excluded_noarg2/", excluded_noarg2),
+    url(r"^response_header/", with_response_header),
     url(r"^span_name/([0-9]{4})/$", route_span_name),
 ]
 _django_instrumentor = DjangoInstrumentor()
@@ -309,3 +320,55 @@ def response_hook(span, request, response):
         self.assertIsInstance(response_hook_args[1], HttpRequest)
         self.assertIsInstance(response_hook_args[2], HttpResponse)
         self.assertEqual(response_hook_args[2], response)
+
+    def test_trace_response_headers(self):
+        response = Client().get("/span_name/1234/")
+        self.assertNotIn("Server-Timing", response._headers)
+        self.memory_exporter.clear()
+
+        set_global_back_propagator(TraceResponsePropagator())
+
+        response = Client().get("/span_name/1234/")
+        span = self.memory_exporter.get_finished_spans()[0]
+
+        self.assertIn("traceresponse", response._headers)
+        self.assertEqual(
+            response._headers["access-control-expose-headers"][0],
+            "Access-Control-Expose-Headers",
+        )
+        self.assertEqual(
+            response._headers["access-control-expose-headers"][1],
+            "traceresponse",
+        )
+        self.assertEqual(
+            response._headers["traceresponse"][0], "traceresponse"
+        )
+        self.assertEqual(
+            response._headers["traceresponse"][1],
+            "00-{0}-{1}-01".format(
+                format_trace_id(span.get_span_context().trace_id),
+                format_span_id(span.get_span_context().span_id),
+            ),
+        )
+        self.memory_exporter.clear()
+
+    def test_trace_response_header_pre_existing_header(self):
+        set_global_back_propagator(TraceResponsePropagator())
+
+        response = Client().get("/response_header/")
+        span = self.memory_exporter.get_finished_spans()[0]
+        self.assertIn("traceresponse", response._headers)
+        self.assertEqual(
+            response._headers["access-control-expose-headers"][1],
+            "X-Test-Header, traceresponse",
+        )
+        self.assertEqual(
+            response._headers["traceresponse"][1],
+            "abc; val=1, "
+            + "00-{0}-{1}-01".format(
+                format_trace_id(span.get_span_context().trace_id),
+                format_span_id(span.get_span_context().span_id),
+            ),
+        )
+
+        self.memory_exporter.clear()

instrumentation/opentelemetry-instrumentation-django/tests/views.py

@@ -1,5 +1,9 @@
 from django.http import HttpResponse
 
+from opentelemetry.instrumentation.propagators import (
+    _HTTP_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS,
+)
+
 
 def traced(request):  # pylint: disable=unused-argument
     return HttpResponse()
@@ -29,3 +33,10 @@ def route_span_name(
     request, *args, **kwargs
 ):  # pylint: disable=unused-argument
     return HttpResponse()
+
+
+def with_response_header(request):  # pylint: disable=unused-argument
+    response = HttpResponse()
+    response["traceresponse"] = "abc; val=1"
+    response[_HTTP_HEADER_ACCESS_CONTROL_EXPOSE_HEADERS] = "X-Test-Header"
+    return response