Metrics are not tools, they're indicators
Decision-making in business is answering the question "What is the next step?". To know that, you need to first figure out where you are as a business at the moment. That's what metrics are for.
What do they indicate? Many things, but the outcome depends on how well you manage metrics and extract the data they hold.
Check out this example.
Feel the difference? Right, context matters.
So remember, there's no universal template. Businesses are unique each by themselves. They're much like living organisms, with millions of different aspects to them: from market specifics, regional and national disposition (like some fast-food chains), scale, corporate identity to staff qualification, business hours and CEO competence. Even ergonomics of a workplace can act as a metric to (for example) define the efficiency of your workforce.
LIFEHACK: for new in-house projects I often just install basic tracking codes (e.g.: Google Analytics) and wait for the first conversions to kick in. Only after that, I do in-depth planning and set up an analytics tool kit.
Analytics and decisions
Metrics and analytics are extremely complex subjects. These subjects are manageable but require time, patience, effort and experience. Analyzing and making solid business decisions is usually impossible for one specialist to handle in medium or large businesses. Decision making usually requires a pair consisting of top-level manager (or CEO) and a proficient analyst, capable of identifying, processing and properly presenting analytic data to a decision-maker. The pair can be extended to a number of more than two specialists. It all depends.
TIP: there is one (but not the only) approach to metrics I like and use to weed out metrics I do not need. I answer a series of questions about a metric. If all answers are positive, I take the metric into consideration.
In my experience it works fine, but, yet again — it is not the only viable approach. There are tons of articles on metrics all around the Internet. Reading almost any material on the subject will eventually pay off, as you will be gradually able to grasp the idea of metrics and data-driven decisions more and more. So, if you entertain the idea of mastering metrics yourself, and feel like a natural born handy-andy, suit yourself.
I'll just give you a brief depiction of what options you have in regard to metrics analytics.
Who: startups & some small businesses
Who: medium to large businesses
Who: most businesses, with few exceptions.
Who: anyone willing to allocate some budget
To sum it up on metrics-driven decisions
Metrics-driven decisions are a crucial factor in the survival of your business. They may propel your business to the greatest of heights as well as ruin it if you do not take metrics seriously. It doesn't mean that your business won't function. It doesn't mean you can't manage your business and drive the profit up (more or less). It only means that without a proper analysis however good you are in your old-school approach, in the modern environment you are bound to be left far behind by your competition.
As a conclusion, it is fair to say the size of your business is directly proportional to the necessity of implementing metrics analysis in your business practices. So choose your options according to your results, needs, and budget.