Add ErrorArg wrapper for Exception and In-place errors

Summary:
Introduce `ErrorArg`, a wrapper class that can be implicitly constructed
from either a `const std::exception&` or a `std::string`.

This allows factory methods to accept both exception and string arguments with a single signature, avoiding templates or overloads. This will be useful when user uses the same function for either logging exception if it is available or just the error message if exception is not available (in-place error handling).

Reviewed By: vilatto

Differential Revision: D96323869

fbshipit-source-id: aefd8e25c84fdc90c642efef3e5e970c92d557ca

Author: Chandhana Solainathan

eden/fs/telemetry/BUCK

@@ -123,6 +123,7 @@ cpp_library(
     headers = [
         "EdenComponent.h",
         "EdenErrorInfo.h",
+        "ErrorArg.h",
     ],
 )
 

eden/fs/telemetry/ErrorArg.h

@@ -0,0 +1,35 @@
+/*
+ * Copyright (c) Meta Platforms, Inc. and affiliates.
+ *
+ * This software may be used and distributed according to the terms of the
+ * GNU General Public License version 2.
+ */
+
+#pragma once
+
+#include <cstdint>
+#include <optional>
+#include <string>
+
+namespace facebook::eden {
+
+/**
+ * Wrapper that can be implicitly constructed from either a std::exception or a
+ * std::string. Extracts error message, and optionally error code and error
+ * name (from std::system_error).
+ */
+class ErrorArg {
+ public:
+  // Implicit conversions — callers can pass either type directly.
+  ErrorArg(const std::exception& ex);
+  ErrorArg(std::string message);
+  ErrorArg(const char* message);
+
+  std::string message;
+  // Numeric errno from std::system_error (e.g. ENOENT=2, EACCES=13).
+  std::optional<int64_t> errorCode;
+  std::optional<std::string> errorName;
+  std::optional<std::string> exceptionType;
+};
+
+} // namespace facebook::eden