feat(logging): add Loggers registry (6/6)

[kamcheungting-db]

Part 6 of the logging stack (builds on #726). Lets applications pick and install a logging backend from configuration instead of writing code — the same pattern as MetricsReporters.

How it works

Choose a backend with the logger-impl property; built-ins are noop, cerr, and spdlog (when compiled in).

// Build a logger from properties and install it as the process default.
iceberg::Loggers::LoadAndSetDefault({{"logger-impl", "spdlog"}, {"level", "info"}});

// ...then anywhere in the code:
ICEBERG_LOG_INFO("scan planned: {} files", n);

API

• Loggers::Load(properties) — build a logger, choosing the type from logger-impl.
• Loggers::LoadAndSetDefault(properties) — build one and install it as the process default.
• Loggers::Register(type, factory) — register a custom backend under a new name.

With no logger-impl set, the default is spdlog when compiled in, otherwise cerr — so logging works out of the box.

Tests — logging_end_to_end_test drives the full path an app uses: configure a backend + level via properties, install it, log through the macros, and check the real output. Covers the default backend and a macro reaching a real spdlog sink, with spdlog ON and OFF.

This pull request and its description were written by Isaac.

[manuzhang]

@kamcheungting-db can you please rebase your PR?

[Copilot]

Pull request overview

This PR completes the logging stack by adding a configuration-driven Loggers registry/factory (mirroring MetricsReporters), installing a process-default logger selected by properties, and introducing public ICEBERG_LOG_* macros (plus opt-in bare LOG_* aliases) with end-to-end and unit test coverage across spdlog-on/off builds.

Changes:

• Added Loggers::{Register, Load, LoadAndSetDefault} with built-in backends (noop, cerr, and spdlog when compiled in) and a compiled-backend default.
• Introduced iceberg/logging/log_macros.h (and short_log_macros.h) plus a FatalHandler hook for fatal logging.
• Added comprehensive tests (macros behavior, registry behavior, spdlog backend, and end-to-end logging output), and wired builds (CMake + Meson) including MSVC /Zc:preprocessor for __VA_OPT__.

Reviewed changes

Copilot reviewed 22 out of 22 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/iceberg/test/spdlog_logger_test.cc	Adds unit tests for the spdlog-backed logger implementation.
src/iceberg/test/meson.build	Registers new logging-related tests in Meson.
src/iceberg/test/macros_test.cc	Adds runtime behavior + death tests for logging macros (formatting, gating, fatal semantics, handler).
src/iceberg/test/macros_active_level_test.cc	Tests compile-time active-level stripping behavior.
src/iceberg/test/logging_end_to_end_test.cc	End-to-end validation of registry/config/default/macro integration and real output.
src/iceberg/test/loggers_test.cc	Adds unit tests for the Loggers registry and property handling.
src/iceberg/test/CMakeLists.txt	Enables MSVC conforming preprocessor for tests; adds new test sources.
src/iceberg/meson.build	Adds logging sources (including registry + spdlog backend) to Meson build.
src/iceberg/logging/short_log_macros.h	Provides opt-in bare LOG_* aliases for ICEBERG_LOG_*.
src/iceberg/logging/meson.build	Installs new public logging headers; generates Meson-only config.h.
src/iceberg/logging/loggers.h	Declares the public Loggers registry/factory API and property keys.
src/iceberg/logging/loggers.cc	Implements the registry, built-in factories, and load/initialize semantics.
src/iceberg/logging/logger.h	Extends logging API docs; adds FatalHandler API surface.
src/iceberg/logging/logger.cc	Implements compiled-backend default selection and fatal handler storage/access.
src/iceberg/logging/log_macros.h	Adds the ICEBERG_LOG_* macro layer and supporting helpers.
src/iceberg/logging/internal/spdlog_logger.h	Introduces internal spdlog-backed SpdLogger sink.
src/iceberg/logging/internal/spdlog_logger.cc	Implements spdlog sink behavior including pattern support and level mapping.
src/iceberg/logging/config.h.in	Adds generated build-time logging backend configuration header template.
src/iceberg/CMakeLists.txt	Generates config.h; gates spdlog compilation/link; exports /Zc:preprocessor for consumers.
meson.build	Enables /Zc:preprocessor via Meson supported-args on MSVC.
CMakeLists.txt	Adds ICEBERG_SPDLOG option.
cmake_modules/IcebergThirdpartyToolchain.cmake	Gates spdlog dependency resolution behind ICEBERG_SPDLOG.