These annotations are not available to the runtime code. You have following options instead:
copy it so that you can access it in the runtime, e.g. using tailf:meta-data or directly in your code - this is obviously acceptable only if you have just a few such leaves;
process the YANG module (in the compile time, presumably) so that you can access the data - transform it to YIN and extract from that, or write a pyang plugin that generates a C-struct or something that you can use in your code.