What to use to measure frontend metrics?

(Photo by Mitchel Boot)

I'm curious what are your, DEV Community, choices for frontend analytics tools. And what I mean by that?

• Let's say I want to check click-rate on links in DEV sidebar (left) nav to see which are used the least (so maybe we can hide some?).
• Let's say I want to measure how often people click links in right sidebar widgets and break it down per each widget (listings, #help, #ama, #challenge, ...). Are some widgets used less than the others?
• Let's say I want to compare if people prefer to click "Write a post" CTA in header OR the one in profile pic dropdown.
• Let's say I want to compare if people prefer to go to own profile by clicking the widget in left sidebar OR the one in profile pic dropdown.
• Let's say I want to see how the onboarding performs - what people click on each step, when do they drop-off, how many people skip some fields etc.

Why? Good design decisions are driven by data. Data also helps validate ideas and its success.

PS. Bonus points if it's open source :).