lg2: Support sdbusplus::message::object_path type

Currently the sdbusplus::message::object_path type is not supported.
To print the sdbusplus::message::object_path type, we need to
explicitly obtain the string.

Resolves openbmc/phosphor-logging#25.

Without this patch:
```
sdbusplus::message::object_path objectPath;
info("objectPath: {PATH}", "PATH", objectPath.str);
```

With this patch:
```
sdbusplus::message::object_path objectPath;
info("objectPath: {PATH}", "PATH", objectPath);
```

Signed-off-by: George Liu <liuxiwei@inspur.com>
Change-Id: Ica2eb19554203af578b8368b4c63c0efc9835857
diff --git a/lib/include/phosphor-logging/lg2/conversion.hpp b/lib/include/phosphor-logging/lg2/conversion.hpp
index 87f5936..cdff0a3 100644
--- a/lib/include/phosphor-logging/lg2/conversion.hpp
+++ b/lib/include/phosphor-logging/lg2/conversion.hpp
@@ -58,6 +58,10 @@
 template <typename T>
 concept exception_type = std::derived_from<std::decay_t<T>, std::exception>;
 
+template <typename T>
+concept sdbusplus_object_path =
+    std::derived_from<std::decay_t<T>, sdbusplus::message::object_path>;
+
 /** Concept listing all of the types we know how to convert into a format
  *  for logging.
  */
@@ -65,7 +69,8 @@
 concept unsupported_log_convert_types =
     !(unsigned_integral_except_bool<T> || std::signed_integral<T> ||
       std::same_as<bool, T> || std::floating_point<T> || string_like_type<T> ||
-      pointer_type<T> || sdbusplus_enum<T> || exception_type<T>);
+      pointer_type<T> || sdbusplus_enum<T> || exception_type<T> ||
+      sdbusplus_object_path<T>);
 
 /** Any type we do not know how to convert for logging gives a nicer
  *  static_assert message. */
@@ -255,6 +260,26 @@
     return std::make_tuple(h, (f | str).value, v.what());
 }
 
+/* Logging conversion for object path. */
+template <log_flags... Fs, sdbusplus_object_path V>
+static auto log_convert(const char* h, log_flag<Fs...> f, V&& v)
+{
+    // Compile-time checks for valid formatting flags.
+    prohibit(f, bin);
+    prohibit(f, dec);
+    prohibit(f, field16);
+    prohibit(f, field32);
+    prohibit(f, field64);
+    prohibit(f, field8);
+    prohibit(f, floating);
+    prohibit(f, hex);
+    prohibit(f, signed_val);
+    prohibit(f, unsigned_val);
+
+    // Treat like a string, but get the 'str' from the object path.
+    return std::make_tuple(h, (f | str).value, v.str);
+}
+
 /** Class to facilitate walking through the arguments of the `lg2::log` function
  *  and ensuring correct parameter types and conversion operations.
  */