It has been raining a lot since you have been here.
Is it OK to add a lot to this sentence because been raining almost constantly? Is it not a pleonasm: the use of more words than are necessary to explain the concept.
Raining a lot is a perfectly acceptable phrase in English.
It is not a pleonasm because it is being used to describe how much it has been raining since the person left.
It has been raining since you left.
This means it hasn't stopped raining since you left.
It has been raining a lot since you left.
It has rained quite often (but not necessarily continuously) since you left.
Side note. I had to look up the word pleonasm to answer you. Your usage of it is perfectly correct, but it is very obscure to most native speakers. Most English speakers would say redundant or extraneous.