client: fix overriding of transport json serializer

We get a string from config so need to import it.

elasticapm/base.py

@@ -155,7 +155,8 @@ def __init__(self, config=None, **inline) -> None:
             "processors": self.load_processors(),
         }
         if config.transport_json_serializer:
-            transport_kwargs["json_serializer"] = config.transport_json_serializer
+            json_serializer_func = import_string(config.transport_json_serializer)
+            transport_kwargs["json_serializer"] = json_serializer_func
 
         self._api_endpoint_url = urllib.parse.urljoin(
             self.config.server_url if self.config.server_url.endswith("/") else self.config.server_url + "/",

tests/client/client_tests.py

@@ -48,6 +48,11 @@
 import elasticapm
 from elasticapm.base import Client
 from elasticapm.conf.constants import ERROR
+
+try:
+    from elasticapm.utils.simplejson_encoder import dumps as simplejson_dumps
+except ImportError:
+    simplejson_dumps = None
 from tests.fixtures import DummyTransport, TempStoreClient
 from tests.utils import assert_any_record_contains
 
@@ -228,6 +233,14 @@ def test_custom_transport(elasticapm_client):
     assert isinstance(elasticapm_client._transport, DummyTransport)
 
 
+@pytest.mark.skipIf(simplejson_dumps is None)
+@pytest.mark.parametrize(
+    "elasticapm_client", [{"transport_json_serializer": "elasticapm.utils.simplejson_encoder.dumps"}], indirect=True
+)
+def test_custom_transport_json_serializer(elasticapm_client):
+    assert elasticapm_client._transport._json_serializer == simplejson_dumps
+
+
 @pytest.mark.parametrize("elasticapm_client", [{"processors": []}], indirect=True)
 def test_empty_processor_list(elasticapm_client):
     assert elasticapm_client.processors == []