KeyValuePair with ContextMapLookup does not resolve variable

[goober]

I am trying to send a variable set at runtime through MDC as a KeyValuePair element by specifying the following element:

<KeyValuePair key="userId" value="$${ctx:userId}"/>

However, the variable cannot be resolved. In the case where I set the variable as part of the PatternLayout it is resolved fine.

What I can find out from the code in GelfAppender.java is that the KeyValuePair elements are added once at creation, but runtime variables are never resolved.

Before I dig into fixing my issue, and creating a pull request I would like to know if this is a known issue, or if there is another way to provide ContextMapLookup variables as KeyValuePair elements.

Note also that variables provided at startup e.g. ${env:APPLICATION_NAME} are resolved just fine