I'm going to assume you've already set up Google Analytics in a standard way to track pageviews for your knowledge base...you already have it set to see # visitors to each page. That's step zero.
I always start by asking the question, "What is the job we hired (the website, or website section) to do?" Then you can go about defining metrics. You've already said it's job is to reduce incoming support requests, so you need to track that. Assuming you take support requests online:
I'd set up an Event every
time someone files a request. You want to see how changes to your
knowledge base makes that number go down.
Many sites have sort of a workflow when filing new requests. First users browse the knowledge base, then go to a form to ask their specific question, then see a confirmation page. In Google Analytics you track a flow through a set of predefined steps with a Goal Funnel
Do you care about segmenting knowledge base/support requestors? Customers vs. non-customers? Logged in vs not-logged in? You can tag users or sessions with custom variables to give you additional segmentation powers.
You may want to look at Google's new Content Grouping feature. That may allow you to track visits across your knowledge base as a single entity, and watch views go up or down
Tracking support that comes in via phone/offline is trickier. But with the Universal Analytics in public beta, you can now upload custom data. So you could really upload CRM or call center data into Google Analytics.