data/db: add counting methods for Artists, Albums, Songs, Folders
api/status: add metrics about wavepipe database
API: add documentation about api/status Metrics object
core/apiRouter: add logging of query parameters
api/status: move Metrics to common/metrics
API: fix Metrics object link
core/apiRouter_test: add api/status?metrics=true test
core/cronManager: add metrics to status logging output
Revert "core/cronManager: add metrics to status logging output"

This reverts commit 7dd490db0c4095719f938be864bdca82431d8b07.
Merge branch 'master' into dev_subsonic_api