@Nullable
    default RequestContextStorageProvider requestContextStorageProvider() {
        return null;
    }
Returns the {@link RequestContextStorageProvider} that provides the {@link RequestContextStorage}.

<p>By default, If no {@link RequestContextStorageProvider} SPI provider implementation is provided,
This flag returns {@link RequestContextStorageProvider} that provides
{@link RequestContextStorage#threadLocal()}. Otherwise, the first {@link RequestContextStorageProvider}
SPI provider implementation will be selected.</p>

<p>By specifying the {@code -Dcom.linecorp.armeria.requestContextStorageProvider=<FQCN>} JVM option, you
are able to select which {@link RequestContextStorageProvider} SPI provider implementation to used.
If none of them matches, the next {@link FlagsProvider#requestContextStorageProvider()} will be
selected.</p>
core/src/main/java/com/linecorp/armeria/common/FlagsProvider.java