Adds link to metrics topic to CF subnav [#155163177]
Adds link to metrics topic to CF subnav [#155163177]

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