Added Mapped Diagnostic Context (MDC) support

[massimiliano96]

Description

This pull request introduces support for Mapped Diagnostic Context (MDC). MDC allows users to associate contextual information with log events, providing valuable insights into specific execution flows.

Changes:

Added mdc_formatter class to handle MDC formatting.
Introduced MDC class to manage contextual information.
Provided test cases to validate MDC functionality.

Usage:

// Setting MDC values
spdlog::mdc::put("mdc_key_1", "mdc_value_1");
spdlog::mdc::put("mdc_key_2", "mdc_value_2");

// Creating a logger with MDC support
auto formatter = std::make_shared<spdlog::pattern_formatter>("\n");
formatter->set_pattern("[%n] [%l] [%&] %v");

// Logging a message
memory_buf_t formatted;
spdlog::details::log_msg msg(spdlog::source_loc{}, "logger-name", spdlog::level::info, "some message");
formatter->format(msg, formatted);

// Expected output: [logger-name] [info] [mdc_key_1:mdc_value_1 mdc_key_2:mdc_value_2] some message

// Removal of an MDC value
spdlog::mdc::remove("mdc_key_2");

Test Cases:

• Validated MDC formatting with multiple values.
• Checked MDC value update.
• Tested MDC behavior across different threads.
• Verified MDC key removal.
• Empty MDC

[massimiliano96]

@gabime Can I do something about this PR?

[gabime]

@massimiliano96 I don't have time to look at it these days, since I am under missile attacks 24x7 launched by the Hamas terrorists.

[massimiliano96]

@gabime I'm really sorry, I had no idea you were in Israel. My thoughts are with you and your family during this terrible situation. Stay safe.

[gabime]

Thanks @massimiliano96 !

[massimiliano96]

Thanks to you @gabime ! I'm going to create another PR for example.cpp and updated docs.

[sketch34]

@massimiliano96 just checking, will this work correctly with async_logger? Since the formatting will occur on a thread in the thread pool which will be different from the thread that spdlog::mdc::put() was called on.

[gabime]

You are right @sketch34. It is not supported in async loggers. I updated the docs.