adds variables for open source content in overview of logging and metrics topic [#161608603]
adds variables for open source content in overview of logging and metrics topic [#161608603]

cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book,cloudfoundry/docs-book-cloudfoundry,swisscom/docs-appcloud-book,swisscom/docs-appcloud-book