One way to do non-global Prometheus metrics in an application

non_global_metrics.go

package main

import (
	"os"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promauto"
	"github.com/prometheus/common/expfmt"
)

// Adder adds numbers.
type Adder struct {
	addsTotal prometheus.Counter
}

// NewAdder instantiates an Adder and registers metrics to the given
// reg. reg may be nil.
func NewAdder(reg prometheus.Registerer) *Adder {
	// If other structs need access to addsTotal, you'd instead want to create an
	// AdderMetrics struct that holds metrics and pass that around instead:
	//
	//	type AdderMetrics struct {
	//		addsTotal prometheus.Counter
	//	}
	//
	//	func NewAdderMetrics(reg prometheus.Registerer) *AdderMetrics {
	//		return &AdderMetrics{
	//			addsTotal: promauto.With(reg).NewCounter(prometheus.CounterOpts{
	//				Name: "adder_adds_total",
	//				Help: "Total amount of times Adder.Add was invoked",
	//			}),
	//		}
	//	}
	//
	return &Adder{
		addsTotal: promauto.With(reg).NewCounter(prometheus.CounterOpts{
			Name: "adder_adds_total",
			Help: "Total amount of times Adder.Add was invoked",
		}),
	}
}

// Add adds two values.
func (a *Adder) Add(lhs, rhs int) int {
	a.addsTotal.Inc()
	return lhs + rhs
}

// Output:
//
//   # HELP adder_adds_total Total amount of times Adder.Add was invoked
//   # TYPE adder_adds_total counter
//   adder_adds_total 100
func main() {
	// You could also use prometheus.DefaultRegistry/DefaultGatherer here if
	// you wanted to keep default metrics (like the Go runtime metrics).
	reg := prometheus.NewPedanticRegistry()

	adder := NewAdder(reg)
	for i := 0; i < 100; i++ {
		adder.Add(i, i)
	}

	printMetrics(reg)
}

func printMetrics(gatherer prometheus.Gatherer) {
	families, err := gatherer.Gather()
	if err != nil {
		panic(err)
	}

	enc := expfmt.NewEncoder(os.Stdout, expfmt.FmtText)
	for _, f := range families {
		if err := enc.Encode(f); err != nil {
			panic(err)
		}
	}
}