subchannel: log status to channelz in state TRANSIENT_FAILURE (#31966)

* subchannel: log status to channelz in state TRANSIENT_FAILURE

* iwyu

src/core/ext/filters/client_channel/subchannel.cc

@@ -59,6 +59,7 @@
 #include "src/core/lib/handshaker/proxy_mapper_registry.h"
 #include "src/core/lib/iomgr/exec_ctx.h"
 #include "src/core/lib/iomgr/pollset_set.h"
+#include "src/core/lib/slice/slice_internal.h"
 #include "src/core/lib/surface/channel_init.h"
 #include "src/core/lib/surface/channel_stack_type.h"
 #include "src/core/lib/surface/init_internally.h"
@@ -799,29 +800,6 @@ void Subchannel::RemoveDataProducer(DataProducerInterface* data_producer) {
   }
 }
 
-namespace {
-
-// Returns a string indicating the subchannel's connectivity state change to
-// \a state.
-const char* SubchannelConnectivityStateChangeString(
-    grpc_connectivity_state state) {
-  switch (state) {
-    case GRPC_CHANNEL_IDLE:
-      return "Subchannel state change to IDLE";
-    case GRPC_CHANNEL_CONNECTING:
-      return "Subchannel state change to CONNECTING";
-    case GRPC_CHANNEL_READY:
-      return "Subchannel state change to READY";
-    case GRPC_CHANNEL_TRANSIENT_FAILURE:
-      return "Subchannel state change to TRANSIENT_FAILURE";
-    case GRPC_CHANNEL_SHUTDOWN:
-      return "Subchannel state change to SHUTDOWN";
-  }
-  GPR_UNREACHABLE_CODE(return "UNKNOWN");
-}
-
-}  // namespace
-
 // Note: Must be called with a state that is different from the current state.
 void Subchannel::SetConnectivityStateLocked(grpc_connectivity_state state,
                                             const absl::Status& status) {
@@ -845,8 +823,10 @@ void Subchannel::SetConnectivityStateLocked(grpc_connectivity_state state,
     channelz_node_->UpdateConnectivityState(state);
     channelz_node_->AddTraceEvent(
         channelz::ChannelTrace::Severity::Info,
-        grpc_slice_from_static_string(
-            SubchannelConnectivityStateChangeString(state)));
+        grpc_slice_from_cpp_string(absl::StrCat(
+            "Subchannel connectivity state changed to ",
+            ConnectivityStateName(state),
+            status.ok() ? "" : absl::StrCat(": ", status_.ToString()))));
   }
   // Notify non-health watchers.
   watcher_list_.NotifyLocked(state, status_);