There are a host of metrics to look at when you have a robust analytics solution, but having too many goals to live up to tends to result in prioritization difficulties. I recommend content marketing teams have 2-3 metrics they measure, and perhaps some secondary metrics each sub-team can measure to help understand when there are different levers to pull. Here are my recommendations:

In some cases, the guidance or existing copy that a content writer receives from his or her client is, in a word, awful. Many are the days that I’ve sat down to look at a wall of ill-formatted, poorly spelled, stream-of-consciousness rough copy, and begin the arduous task of editing and polishing it. It’s the intellectual equivalent of cleaning up someone else’s bedroom: you just don’t want to know what goes on in there to cause such a mess. Still, we do it, and we smile, because in the end making you look and sound good is what our jobs are all about.

Have you ever heard the phrase, “Sell a good night’s sleep, not the mattress”? Think about all of the mattress commercials and advertisements you’ve seen before. Do they lead with coil count or insulation padding details? Nah. They sell you on how comfortable the mattress is, how well you’ll sleep on it, and how much more productive and enjoyable your days will be when you are well-rested. They want you to positively frame their product, envision yourself using it, and negate any potential concerns you may have.
Problem: I need to create brand preference. Engaging content marketing creates preference through thought leadership by making you a trusted source of information and education. You can also create preference through relationships, which are strengthened whenever your content entertains or helps your buyers. People are more likely to buy from companies with whom they have relationships. 
While successful content writers seem to have an enviable life -- they work from home, make their own schedules and work as much or as little as they please -- the vast majority have a hard time making a living of it. They lack the skills necessary to succeed. Because no matter how talented they are, writing skill is simply not enough. So, if you want to become successful as a content writer, you need a full toolkit of marketable skills.