MetatagManager::getFields() is unnecessarily slow

The function MetatagManager::getFields() takes about 23ms on the EvolvingWeb site. It's approximately linear on the number of fields, and the entity in question only has three fields, so it's probably worse on other sites.

It looks like the function is fetching the values of all fields, in order to check if they are metatag fields. But we don't need the values for that, just the definitions! So it should be possible to greatly speed it up.