Does not work for other CachingBackends

[mgrundkoetter]

The data collectors are only useful in combination with the Typo3DatabaseBackend. If you use something else, like the RedisBackend, the values are still being collected, but they are quite useless, as the db tables do not get changed by TYPO3 any more.
So if, for instance, this is set:

$GLOBALS['TYPO3_CONF_VARS']['SYS']['caching']['cacheConfigurations']['cache_pages']['backend'] = 'TYPO3\\CMS\\Core\\Cache\\Backend\\RedisBackend';

the CfCachePagesMetrics has to query the redis backend for data, not the TYPO3 database. For the redis case, there is dbsize. For other caching backends like apcu or memcached, there are corresponding methods as well.