refactor(logging): capture Builder source_location at the caller's site

Take std::source_location as a defaulted constructor argument so Builder(level)
records the caller's file/line by default; current() in the init-list would
record logger.h instead. Add tests for the default caller-site location and the
Location() override.

Co-authored-by: Isaac

Author: kamcheungting-db

src/iceberg/logging/logger.h

@@ -81,11 +81,14 @@ struct ICEBERG_EXPORT LogMessage {
 ///                     .Build();
 ///   logger->Log(std::move(record));
 ///
-/// The location defaults to where the Builder is constructed; override it with
-/// Location() (e.g. to forward a caller's std::source_location).
+/// The location defaults to the caller's construction site (captured via the
+/// constructor's default argument); override it with Location() (e.g. to forward
+/// a caller's std::source_location).
 class ICEBERG_EXPORT LogMessage::Builder {
  public:
-  explicit Builder(LogLevel level) : level_(level), location_(std::source_location::current()) {}
+  explicit Builder(LogLevel level,
+                   std::source_location location = std::source_location::current())
+      : level_(level), location_(location) {}
 
   /// \brief Set the already-formatted message text.
   Builder& Message(std::string message) {

src/iceberg/test/logger_test.cc

@@ -21,7 +21,6 @@
 
 #include <atomic>
 #include <memory>
-#include <string_view>
 #include <thread>
 #include <tuple>
 #include <vector>
@@ -430,16 +429,14 @@ TEST(LoggerTest, BuilderDefaultsAndEmitToSink) {
   EXPECT_NE(records[0].location.line(), 0u);  // location defaulted at build site
 }
 
-// location_ is initialized in the Builder constructor, so when Location() is not
-// called the default points at the constructor itself (in logger.h), not at the
-// caller and not at line 0.
-TEST(LoggerTest, BuilderDefaultLocationIsConstructorSite) {
+// The constructor captures source_location as a default argument, so without
+// Location() the default is the caller's construction site (this file), not
+// logger.h. The Builder is constructed exactly one line below `here`.
+TEST(LoggerTest, BuilderDefaultLocationIsCallerSite) {
+  auto here = std::source_location::current();
   auto record = LogMessage::Builder(LogLevel::kInfo).Message("m").Build();
-  EXPECT_NE(record.location.line(), 0u);
-  EXPECT_NE(std::string_view(record.location.file_name()).find("logger.h"),
-            std::string_view::npos);
-  EXPECT_NE(std::string_view(record.location.function_name()).find("Builder"),
-            std::string_view::npos);
+  EXPECT_STREQ(record.location.file_name(), here.file_name());
+  EXPECT_EQ(record.location.line(), here.line() + 1);
 }
 
 // Location() replaces the constructor default with the caller's site (file + line).