Reimplement the keys method in WSGI CarrierGetter (#379)

Author: marcinzaremba

Diff for: CHANGELOG.md

@@ -9,6 +9,8 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
   ([#364](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/364))
 - `opentelemetry-propagator-ot-trace` Do not throw an exception when headers are not present
   ([#378](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/378))
+- `opentelemetry-instrumentation-wsgi` Reimplement `keys` method to return actual keys from the carrier instead of an empty list.
+  ([#379](https://github.com/open-telemetry/opentelemetry-python-contrib/pull/379))
 
 ### Changed
 - Rename `IdsGenerator` to `IdGenerator`

Diff for: instrumentation/opentelemetry-instrumentation-wsgi/src/opentelemetry/instrumentation/wsgi/__init__.py

@@ -66,6 +66,8 @@ def hello():
 from opentelemetry.trace.status import Status, StatusCode
 
 _HTTP_VERSION_PREFIX = "HTTP/"
+_CARRIER_KEY_PREFIX = "HTTP_"
+_CARRIER_KEY_PREFIX_LEN = len(_CARRIER_KEY_PREFIX)
 
 
 class CarrierGetter(DictGetter):
@@ -89,7 +91,11 @@ def get(
         return None
 
     def keys(self, carrier):
-        return []
+        return [
+            key[_CARRIER_KEY_PREFIX_LEN:].lower().replace("_", "-")
+            for key in carrier
+            if key.startswith(_CARRIER_KEY_PREFIX)
+        ]
 
 
 carrier_getter = CarrierGetter()

Diff for: instrumentation/opentelemetry-instrumentation-wsgi/tests/test_getter.py

@@ -22,15 +22,30 @@ def test_get_none(self):
         getter = CarrierGetter()
         carrier = {}
         val = getter.get(carrier, "test")
+
         self.assertIsNone(val)
 
-    def test_get_(self):
+    def test_get(self):
         getter = CarrierGetter()
         carrier = {"HTTP_TEST_KEY": "val"}
         val = getter.get(carrier, "test-key")
+
         self.assertEqual(val, ["val"])
 
     def test_keys(self):
+        getter = CarrierGetter()
+        keys = getter.keys(
+            {
+                "HTTP_TEST_KEY": "val",
+                "HTTP_OTHER_KEY": 42,
+                "NON_HTTP_KEY": "val",
+            }
+        )
+
+        self.assertEqual(keys, ["test-key", "other-key"])
+
+    def test_keys_empty(self):
         getter = CarrierGetter()
         keys = getter.keys({})
+
         self.assertEqual(keys, [])