AthenaMonitoring - allow extraction of strings from collections

Per @rbielski request added feature to be able to extract strings from come objects and use them for monitoring. Related to ATLASDQ-727 The test was created in special way. The filling code is placed in a separate file to stand as documentation. Tagging @fwinkl, @ponyisi @cburton on that.

I (and an experienced colleague) spent today morning trying to use the feature of doxygen as it seems ideally suited for sharing code for UT and documentation: http://www.doxygen.nl/manual/commands.html#cmdexample However, I was not able to set EXAMPLE_PATH in the Doxygen config file our build uses. I tried just setting the variable as documented here: https://cmake.org/cmake/help/v3.9/module/FindDoxygen.html (ignored) as well as a brute force injection of "EXAMPLE_PATH=bla" using "file(APPEND ..". (I think the Doxyfile is generated to late and overwrites my "append").