metrics/handler_options.go

package metrics

import (
	"context"

	"github.com/prometheus/client_golang/prometheus"
	"github.com/prometheus/client_golang/prometheus/promhttp"
)

// LabelValueFromContext is used to compute the label value from request context.
// Context can be filled with values from request through middleware.
// This matches promhttp.LabelValueFromCtx.
type LabelValueFromContext func(ctx context.Context) string

type labelValueFromContext struct {
	name    string
	valueFn LabelValueFromContext
}

type handlerConfig struct {
	// labelValues contains static label values.
	labelValues map[string]string

	// labelValuesFromContext contains dynamic label values
	// which can be set by pull information from the
	// context.
	labelValuesFromContext []labelValueFromContext
}

// HandlerOption is used to pass options to the HandlerFactory instance.
type HandlerOption func(*handlerConfig)

// applyHandlerOptions applies the options,
// returns the config, plus a slice of promhttp.Option which can be used
// with the prometheus handlers.
func applyHandlerOptions(opts []HandlerOption) (handlerConfig, []promhttp.Option) {
	config := handlerConfig{}
	for _, v := range opts {
		v(&config)
	}

	var promOpts []promhttp.Option

	// Add prometheus options for labels from context
	for _, v := range config.labelValuesFromContext {
		promOpts = append(promOpts, promhttp.WithLabelFromCtx(v.name, promhttp.LabelValueFromCtx(v.valueFn)))
	}

	return config, promOpts
}

// applyOptionsToCounterVec will apply the options to a CounterVec instance.
func (c *handlerConfig) applyOptionsToCounterVec(cv *prometheus.CounterVec) *prometheus.CounterVec {
	if len(c.labelValues) == 0 {
		return cv
	}

	return cv.MustCurryWith(c.labelValues)
}

// applyOptionsToObserverVec will apply the options to an ObserverVec instance.
func (c *handlerConfig) applyOptionsToObserverVec(cv prometheus.ObserverVec) prometheus.ObserverVec {
	if len(c.labelValues) == 0 {
		return cv
	}

	return cv.MustCurryWith(c.labelValues)
}

// WithLabelValues will configure labels values to apply to this handler.
func WithLabelValues(labelValues map[string]string) HandlerOption {
	return func(config *handlerConfig) {
		config.labelValues = labelValues
	}
}

// WithLabelFromContext will configure labels values to apply to this handler,
// allowing for dynamic label values to be added to the standard metrics.
// This is equivalent to promhttp.WithLabelFromCtx.
func WithLabelFromContext(name string, valueFn LabelValueFromContext) HandlerOption {
	return func(config *handlerConfig) {
		config.labelValuesFromContext = append(config.labelValuesFromContext, labelValueFromContext{name, valueFn})
	}
}