This might seem a bit like a "what's the meaning of life" style question, but I think in many regards it's actually quite a bit simpler. Sure, the nuance will vary from one person to another, but I think certain universals could largely gain consensus.
Although the priority and magnitude of each would require adjusting to suit different situations, my suggestions for metrics which when measured can give an indication of quality of life, are as follows:
There is a lot of room for discussion here, and almost everyone would have a different number in mind when asked what salary would allow this. Marital status, geographic location, number of children, hobbies, homeowner status, age and many other factors could lead to wildly different answers. All of that though, is minutiae. The fact is, not having to fret about bills, or having to check your balance before going shopping, allow us the freedom to focus on a life that money facilitates, rather than focusing on money itself.
I'm not talking mile run times or deadlift numbers here, but a high enough level of fitness to satisfy three criteria; the ability to perform daily tasks with minimal discomfort or effort, the ability to enjoy hobbies and interests of your choosing, and the ability to survive emergency situations you might find yourself exposed to.
There are broadly two criteria covered here; freedom from health conditions brought about by poor metabolic function, or genetic conditions which would ordinarily be considered chronic, and freedom from sickness or injury, which could be considered acute. There are afflictions which sit outside of these definitions, but since we're not in the business of taxonomy, these classifications will suffice for our purposes.
Purpose [ful Work]
There are a very select few people who have a purpose which is so very clear, that they never end up working. In recent years, these tend to be the filthy rich, who find purpose in philanthropy or innovation. For the overwhelming majority of us, however, we have work. For an adult in the United Kingdom, a full working life will start between 18 and 21, and end somewhere between 60 and 70. This is well over half of most people's lifetime, and therefore having or seeing purpose in your work is most definitely of huge importance. Philosophers have debated this point throughout the ages, and I'm sure will continue to do so, however I feel in this case, anecdotal evidence will prove more useful. Have you ever been greeted by someone at the cinema, in a shop or maybe on a hotel reception, who was clearly destined to be in a customer service role? Perhaps you can recall a children's TV presenter with an ever present smile, or a care home worker with a naturally gentle manner? All of these people see purpose in work that you or I may find unappealing, or menial and dull. Truth be told, all of our jobs are far from ideal for the majority, but that's irrelevant. What matters is that you see purpose in the work you do.
The spectrum covered by relationships is actually pretty broad. Research consistently shows that depth of relationships matters far more than the number we have, and that real, in person interactions outweigh those conducted over the telephone, which in turn are more meaningful and rewarding than those conducted over email or text message. Susan Pinker's book "The Village Effect" examines the correlation between these deeply meaningful relationships and length and quality of life, whereas Ray Dalio in "Principles" places meaningful relationships in all aspects of life above financial wealth or success in business, whilst acknowledging that those relationships actually contribute to both.
Work Life Balance
This phrase has been tortured over recent years, largely because until relatively recently, it just wasn't a concern. With more and more technology being introduced into the mainstream, that is supposed to make us more efficient, we still collectively work longer and longer hours. Whilst household chores have been reduced by many hours per week through the introduction of washing and cleaning technologies, and record keeping, archiving and communication have all been sped up beyond our wildest dreams, we seem to only use all of that additional time to work, often for other people, and often for no additional benefits. There seem to be two solutions; striking a balance of time between work and recreation, or to do a job that includes recreation, and where you're surrounded by people you would choose to spend time with anyway. This may sound like a far flung dream, but I can report from personal experience, it is most definitely not!
So all of this begs the question, how do we achieve better quality of life? I've given a couple of quick suggestions so far, but nothing that would drastically change your life. I've also deliberately and noticeably missed out some critical measurements that you may have liked to have seen included. Happiness, access to education and healthcare, absence of war, equal rights, ability to freely practice religion, protection from crime by law, and many others. Whilst I fully agree that these are all worthwhile measurements for the epidemiologists to consider, they have no place on my list for one simple reason; they aren't controllable at the individual level. Happiness is a little different, that's a measurement of the quality of the other things I have mentioned. The others are all implemented at a societal level, and therefore our individual energy would be more efficiently directed at the controlables.
This may come as no surprise from a gym owner, and I fully recognise my bias here, but this list can be overwhelmingly influenced by three things; sleep, diet and exercise. When it comes to fitness and health, these may seem self-explanatory, however, I believe that done right, the benefits run much deeper. If we take work life balance to start with, I think that training regularly is the start of that balance, but also the physical health gained from it can help you to further enjoy your recreation time. Additionally, a healthy sleep routine leaves us fresher to enjoy our recreation time. Take this a step further, let's look at relationships. Sleep influences hormones more than any other single factor, so is it fair to say a well rested person is more likely to form better relationships through better mood, and being more willing to converse? I think so. Is it fair to say that someone who is healthy in body and mind, through sleep diet and exercise, usually has a better self-image and self confidence, meaning they are more enjoyable to be around, and that those relationships are happier ones, as opposed to self-deprecating echo chambers that further compound negative self-images? Again, I think so.
So what about work? Well, I'll openly admit the link is a little more tenuous here, but I still strongly contend that it exists! Healthier body and mind allows better concentration, and ultimately improves performance at work. This will lead to being more respected by peers and superiors alike, and greater job satisfaction; because you will feel like you're having an impact - you'll have a sense of purpose. These people are those who tend to get promoted. It doesn't take a genius to link financial freedom to a better paid position at work. From here, I don't see it as much of a stretch to say a better performer at work, with a sound reputation, has more freedom in the job market to chase the work life balance they desire. I'm not arguing that eating well and lifting weights is the answer to all of this, but I am definitely saying that the positive impacts can be seen under all of these headings, and many more. The second and third order effects of being physically fit and healthy are not only positive, but are becoming increasingly rare in a society where many choose to forego one or both in pursuit of money, quick fixes and hedonic rewards.