expose DBus interface method names as symbols

Enable a way to access the method names of a given DBus interface as a
constexpr symbol via the header.

4 use-cases:

- printing error / debug logs with the method name.
  e.g. 'error calling ${METHOD_NAME}'

- accessing DBus methods in applications which do not yet use the PDI
  generated bindings.

- preventing typos in DBus method names

- estimating the impact of removing a given DBus method from an
  interface. When using these symbols, it would cause a build failure in
  applications relying on the existence of that method.

This change is similar to [1] and goes into the same direction.

Tested: Newly written unit test passes.

References:
[1] d2571922bfdb4f6b41ba4fbc45b8a4272793fd40

Change-Id: Id423c3a668dd1a8346040f4380476d5e02468ddb
Signed-off-by: Alexander Hansen <alexander.hansen@9elements.com>
diff --git a/test/gen/test_method_names.cpp b/test/gen/test_method_names.cpp
new file mode 100644
index 0000000..f9145a1
--- /dev/null
+++ b/test/gen/test_method_names.cpp
@@ -0,0 +1,31 @@
+#include "server/TestWithMethod/common.hpp"
+
+#include <print>
+
+#include <gtest/gtest.h>
+
+TEST(MethodNames, TestMethodNames)
+{
+    // We can access the method name as a symbol.
+    // The property name is constexpr.
+
+    constexpr auto methodName =
+        sdbusplus::common::server::TestWithMethod::method_names::update_value;
+
+    // The method name can be used as part of error logs.
+
+    std::println("error calling method {}\n", methodName);
+
+    // If the method is removed from the interface definition, it will cause a
+    // build failure in applications still using that method. That can work
+    // even if the application is not (yet) using PDI-generated bindings for
+    // it's DBus interactions.
+
+    std::println(
+        "using method {} \n",
+        sdbusplus::common::server::TestWithMethod::method_names::update_value);
+
+    EXPECT_EQ(
+        sdbusplus::common::server::TestWithMethod::method_names::update_value,
+        "UpdateValue");
+}