Isn't "to have met you" a noun phrase (an infinitive phrase)? A clause requires a finite verb, and an infinitive ("to have") is a nonfinite verb.
Anyway, "to have met you" modifies the adjective "glad", so I guess it could be seen as a kind of adverb. Unfortunately, these grammatical terms can be really confusing. As Wikipedia points out here: "Object complements can often be removed leaving a well-formed sentence, thus the use of the term complement is slightly illogical." So, I guess the so-called "adjective complement" works the same way.
Besides that, it wouldn't be an adjunct because (according to Wikipedia) "an adjunct is a sentence element that establishes the circumstances in which the action or state expressed by the verb take place." So, unlike an adverb, an adjunct only modifies verbs, not adjectives.
I agree with you that it shouldn't be called a complement, but that's what the experts decided to call it.
EDIT: I had pointed out that removing "to have met you" would completely change the meaning of the sentence, but I realized (silly me) that removing any adjective or adverb would do the same.
So, "to have met you" isn't a complement in the strictest sense, but the term "complement" is used to describe more than one thing. Again, to quote Wikipedia: "In grammar the term complement is sometimes used with different meanings. . . . We find complements which function as a sentence element (i.e. of equal status to subjects and objects) and complements which exist within sentence elements."