I'm trying to validate my concept of Rationality. Rational behavior as I understand is the inherent nature of beings. Like the need to worship deities or the attribute of self esteem the human psyche. So if that is right, would I encounter this in metaphysics?
I think that rather than Rationality, what you are after is Rationalism. It is quite easy to be quite rational without being rationalistic.
Rationalistic metaphysics tries to motivate all understanding, or at least as large a part as is in any way possible entirely from intuition and logic, and to rely upon experience as little as possible. There are in fact a wide range of deeply rationalistic forms of metaphysics, and it may constitute a majority of the subject historically. So yes, you will find it there.
Such metaphysics generally automatically assume human rational psychology is simply correct, so they tend to favor concepts like God, and the value of autonomy and self-respect, which seem to come naturally to humans, even if they end up not fitting into experience very well. (If we turn starkly toward experience, we become used to the notion of cause and effect, which tends to make us feel mechanical and unfavored, and often not really autonomous, or acquainted with God.)
If a pointer toward these systems is in fact what you are asking, I would suggest that idealists such as Spinoza or Berkely are the most basic examples of deeply rationalistic philosophies, but I would say the best example is Kant, who works very hard at accommodating our ability to learn from experience while refusing to allow encountered reality to ever contravene rationalism.
Rational means many things.
One can use the common-sense understanding of the word rational - if one is religious, say a Christian or a Muslim, then it would be rational to worship the deity.
Rationalism in the history of Western philosophy, given your interest in metaphysics, is one term that is used to describe a way of looking at Being as the necessary Being; Spinozas system is called rational for this reason, and he is called a rationalist; a more descriptive term would be onto-theology pioneered, I think by Heidegger - who wanted to move away from this notion of Being; it's also surprising, since in today's secular society a rationalist would be more akin to an athiest - which Spinoza wasn't.
Metaphysics is a set of lecture notes by Aristotle, and as Heidegger pointed out has very little to do with the mystical sense of metaphysics, and much more to do about thinking through and about physical concepts.