fix: setup middleware without client argument for starlette (#1952)

.ci/.matrix_exclude.yml

@@ -279,3 +279,5 @@ exclude:
     FRAMEWORK: pyodbc-newest  # error on wheel
   - VERSION: python-3.12
     FRAMEWORK: cassandra-newest  # c extension issue
+  - VERSION: python-3.12
+    FRAMEWORK: starlette-newest  # waiting for 3.12.2 for this fix: https://github.com/python/cpython/pull/111221

elasticapm/contrib/starlette/__init__.py

@@ -105,7 +105,7 @@ class ElasticAPM:
     >>> elasticapm.capture_message('hello, world!')
     """
 
-    def __init__(self, app: ASGIApp, client: Optional[Client], **kwargs) -> None:
+    def __init__(self, app: ASGIApp, client: Optional[Client] = None, **kwargs) -> None:
         """
 
         Args:

tests/contrib/asyncio/starlette_tests.py

@@ -534,3 +534,11 @@ def test_transaction_active_in_base_exception_handler(app, elasticapm_client):
         assert exc.transaction_id
 
     assert len(elasticapm_client.events[constants.TRANSACTION]) == 1
+
+
+def test_middleware_without_client_arg():
+    with mock.patch.dict("os.environ", {"ELASTIC_APM_SERVICE_NAME": "foo"}):
+        app = Starlette()
+        elasticapm = ElasticAPM(app)
+
+    assert elasticapm.client.config.service_name == "foo"