SpringBoot ThreadLocalAccessor implementation for MDC

To support MDC propagation between threadlocal and reactor operation chain, e.g. using WebClient in Spring MVC, I use a custom ThreadLocalAccessor(from micrometer context-propagation) implementation.

It would be more useful if it is provided by Spring Boot and auto-configures.

Implementation sample from this gist by @chemicL

    static class MdcAccessor implements ThreadLocalAccessor<Map<String, String>> {

        static final String KEY = "mdc";

        @Override
        public Object key() {
            return KEY;
        }

        @Override
        public Map<String, String> getValue() {
            return MDC.getCopyOfContextMap();
        }

        @Override
        public void setValue(Map<String, String> value) {
            MDC.setContextMap(value);
        }

        @Override
        public void reset() {
            MDC.clear();
        }
    }

Comment From: wilkinsona

Thanks for the suggestion, @ttddyy. I don't see anything specific to Spring Boot here. Would the code above not be better in Micrometer as has been done with LogbackMetrics? We could then perhaps consider some auto-configuration but I don't think the underlying implementation needs to be a Spring Boot feature.

Comment From: rstoyanchev

I can also see this potentially in the context-propagation library itself in a support package with commonly used ThreadLocalAccessor and/or ContextAccessor implementations that don't have other obvious places to exist.

Comment From: marcingrzejszczak

@chemicL WDYT about adding a SLF4j module in context-propagation library?

Comment From: chemicL

Yep, created: https://github.com/micrometer-metrics/context-propagation/issues/191

Comment From: jonatan-ivanov

I think this issue can be closed in favor of the one in context-propagation (https://github.com/micrometer-metrics/context-propagation/issues/191), we can open an new issue/PR once auto-configuration is needed.

Comment From: ttddyy

closing in favor of https://github.com/micrometer-metrics/context-propagation/issues/191