add documentation to Metrics class, refs #4452

git-svn-id: svn://develop.studip.de/studip/trunk@31200 c5b85792-5220-0410-ad23-ed5b6b8ad999
measure every page's request time, refs #4452

git-svn-id: svn://develop.studip.de/studip/trunk@31201 c5b85792-5220-0410-ad23-ed5b6b8ad999
sample only every tenth request time, refs #4452

git-svn-id: svn://develop.studip.de/studip/trunk@31202 c5b85792-5220-0410-ad23-ed5b6b8ad999
consider sample rate in Metrics class, refs #4452

git-svn-id: svn://develop.studip.de/studip/trunk@31203 c5b85792-5220-0410-ad23-ed5b6b8ad999