flask: using string keys for wsgi environ values (#366)

Some WSGI servers (such as Gunicorn) expect keys in the environ object
to be strings.

ext/opentelemetry-ext-flask/src/opentelemetry/ext/flask/__init__.py

@@ -12,9 +12,9 @@
 
 logger = logging.getLogger(__name__)
 
-_ENVIRON_STARTTIME_KEY = object()
-_ENVIRON_SPAN_KEY = object()
-_ENVIRON_ACTIVATION_KEY = object()
+_ENVIRON_STARTTIME_KEY = "opentelemetry-flask.starttime_key"
+_ENVIRON_SPAN_KEY = "opentelemetry-flask.span_key"
+_ENVIRON_ACTIVATION_KEY = "opentelemetry-flask.activation_key"
 
 
 def instrument_app(flask):

ext/opentelemetry-ext-flask/tests/test_flask_integration.py

@@ -14,7 +14,7 @@
 
 import unittest
 
-from flask import Flask
+from flask import Flask, request
 from werkzeug.test import Client
 from werkzeug.wrappers import BaseResponse
 
@@ -57,6 +57,25 @@ def hello_endpoint(helloid):
         otel_flask.instrument_app(self.app)
         self.client = Client(self.app, BaseResponse)
 
+    def test_only_strings_in_environ(self):
+        """
+        Some WSGI servers (such as Gunicorn) expect keys in the environ object
+        to be strings
+
+        OpenTelemetry should adhere to this convention.
+        """
+        nonstring_keys = set()
+
+        def assert_environ():
+            for key in request.environ:
+                if not isinstance(key, str):
+                    nonstring_keys.add(key)
+            return "hi"
+
+        self.app.route("/assert_environ")(assert_environ)
+        self.client.get("/assert_environ")
+        self.assertEqual(nonstring_keys, set())
+
     def test_simple(self):
         expected_attrs = expected_attributes(
             {