Add callback for direct instrument

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

@@ -173,13 +173,18 @@ def _teardown_request(exc):
 
 
 class _InstrumentedFlask(flask.Flask):
+
+    name_callback = get_default_span_name
+
     def __init__(self, *args, **kwargs):
         super().__init__(*args, **kwargs)
 
         self._original_wsgi_ = self.wsgi_app
         self.wsgi_app = _rewrapped_app(self.wsgi_app)
 
-        _before_request = _wrapped_before_request(get_default_span_name)
+        _before_request = _wrapped_before_request(
+            _InstrumentedFlask.name_callback
+        )
         self._before_request = _before_request
         self.before_request(_before_request)
         self.teardown_request(_teardown_request)
@@ -194,6 +199,9 @@ class FlaskInstrumentor(BaseInstrumentor):
 
     def _instrument(self, **kwargs):
         self._original_flask = flask.Flask
+        name_callback = kwargs.get("name_callback")
+        if callable(name_callback):
+            _InstrumentedFlask.name_callback = name_callback
         flask.Flask = _InstrumentedFlask
 
     def instrument_app(

instrumentation/opentelemetry-instrumentation-flask/tests/test_programmatic.py

@@ -208,3 +208,32 @@ class TestProgrammaticCustomSpanName(
         span_list = self.memory_exporter.get_finished_spans()
         self.assertEqual(len(span_list), 1)
         self.assertEqual(span_list[0].name, "flask-custom-span-name")
+
+
+class TestProgrammaticCustomSpanNameCallbackWithoutApp(
+    InstrumentationTest, TestBase, WsgiTestBase
+):
+    def setUp(self):
+        super().setUp()
+
+        def custom_span_name():
+            return "instrument-without-app"
+
+        FlaskInstrumentor().instrument(name_callback=custom_span_name)
+        from flask import Flask
+
+        self.app = Flask(__name__)
+
+        self._common_initialization()
+
+    def tearDown(self):
+        super().tearDown()
+        with self.disable_logging():
+            FlaskInstrumentor().uninstrument()
+
+    def test_custom_span_name(self):
+        self.client.get("/hello/123")
+
+        span_list = self.memory_exporter.get_finished_spans()
+        self.assertEqual(len(span_list), 1)
+        self.assertEqual(span_list[0].name, "instrument-without-app")