Fix max span handling in gRPC client interceptor (#1861)

Co-authored-by: Colton Myers <[email protected]>

Author: ckoehn

CHANGELOG.asciidoc

@@ -37,9 +37,9 @@ endif::[]
 ==== Features
  * Add `server_ca_cert_file` option to provide custom CA certificate {pull}1852[#1852]
 
-// [float]
-// ===== Bug fixes
-
+[float]
+===== Bug fixes
+* Fix a bug in the GRPC instrumentation when reaching the maximum amount of spans per transaction {pull}1861[#1861]
 
 [[release-notes-6.x]]
 === Python Agent version 6.x

elasticapm/contrib/grpc/client_interceptor.py

@@ -35,7 +35,7 @@
 
 import elasticapm
 from elasticapm.conf import constants
-from elasticapm.traces import Span
+from elasticapm.traces import DroppedSpan, SpanType
 from elasticapm.utils import default_ports
 
 
@@ -193,8 +193,8 @@ def intercept_unary_unary(self, continuation, client_call_details, request):
     #       Future interface, though it may not.
     #     """
 
-    def attach_traceparent(self, client_call_details: _ClientCallDetails, span: Span):
-        if not span.transaction:
+    def attach_traceparent(self, client_call_details: _ClientCallDetails, span: SpanType) -> _ClientCallDetails:
+        if isinstance(span, DroppedSpan) or not span.transaction:
             return client_call_details
         meta = list(client_call_details.metadata) if client_call_details.metadata else []
         if constants.TRACEPARENT_HEADER_NAME not in meta:

tests/contrib/grpc/grpc_client_tests.py

@@ -219,6 +219,16 @@ def test_grpc_client_unsampled_transaction(instrument, sending_elasticapm_client
     assert len(payloads) == 1  # only the server_version request
 
 
+@pytest.mark.parametrize("sending_elasticapm_client", [{"transaction_max_spans": 1}], indirect=True)
+def test_grpc_client_max_spans(instrument, sending_elasticapm_client, grpc_client_and_server_url):
+    grpc_client, _ = grpc_client_and_server_url
+    transaction = sending_elasticapm_client.begin_transaction("request")
+    _ = grpc_client.GetServerResponse(Message(message="foo"))
+    _ = grpc_client.GetServerResponse(Message(message="bar"))
+    sending_elasticapm_client.end_transaction("grpc-test")
+    assert transaction.dropped_spans == 1
+
+
 def extract_events_from_payload(service_name, payloads):
     payloads = [
         payload for payload in payloads if payload and payload[0]["metadata"]["service"]["name"] == service_name