do I have to read the source for each object to learns its methods?
Yes, you will have to read the source code to see what methods are available.
Starting with 1.6.0 you can use plugins.fireHook('filter:meta.getMetaTags', { req: req, data: data, defaultTags: defaultTags }, next);
If you want to do it on 1.5.2 you can try the below using the the hook filter:topic.build
.
myPluign.onTopicBuild = function (hookData, callback) {
hookData.res.locals.metaTags.push({ property: 'tags', content: 'abc, def' });
callback(null, hookData);
};