Keep the I/O error logs at debug

Recording a completion is first writer wins, so a read or write error that
arrives after a response has already succeeded does not change the result
Export() reports. Logging it at error level announced a failure the caller
was never told about.

Destroyed is the existing precedent in this switch: it ends the wait the
same way and stays at debug for the same reason.

Covered by IoErrorAfterAResponseKeepsTheSuccess.

Author: thc1006

exporters/elasticsearch/src/es_log_record_exporter.cc

@@ -182,11 +182,14 @@ class ResponseHandler : public http_client::EventHandler
         recordCompletion(CompletionState::Failure);
         break;
       case http_client::SessionState::ReadError:
-        OTEL_INTERNAL_LOG_ERROR("[ES Log Exporter] Read error");
+        OTEL_INTERNAL_LOG_DEBUG("[ES Log Exporter] Read error");
+        // Kept at debug, like Destroyed above: recording is first writer wins, so this can
+        // arrive after a response has already succeeded, and an error line there would not
+        // describe the outcome Export() reports.
         recordCompletion(CompletionState::Failure);
         break;
       case http_client::SessionState::WriteError:
-        OTEL_INTERNAL_LOG_ERROR("[ES Log Exporter] Write error");
+        OTEL_INTERNAL_LOG_DEBUG("[ES Log Exporter] Write error");
         recordCompletion(CompletionState::Failure);
         break;
       case http_client::SessionState::Cancelled:

exporters/elasticsearch/test/es_log_record_exporter_test.cc

@@ -311,3 +311,20 @@ TEST(ElasticsearchLogsExporterSyncTests, SessionDestroyedAfterAResponseKeepsTheS
   });
   EXPECT_EQ(result, opentelemetry::sdk::common::ExportResult::kSuccess);
 }
+
+// Same rule for the two states this change makes terminal. It is why they log at debug rather
+// than error: reaching one of them says nothing about the result Export() goes on to report.
+TEST(ElasticsearchLogsExporterSyncTests, IoErrorAfterAResponseKeepsTheSuccess)
+{
+  SKIP_WITHOUT_SYNC_EXPORT();
+  for (const auto state : {http_client::SessionState::ReadError,
+                           http_client::SessionState::WriteError})
+  {
+    const auto result = ExportWith([state](http_client::EventHandler &handler) {
+      FakeResponse response(200, kAcceptedBody);
+      handler.OnResponse(response);
+      handler.OnEvent(state, "");
+    });
+    EXPECT_EQ(result, opentelemetry::sdk::common::ExportResult::kSuccess);
+  }
+}