Metrics interface and Prometheus implementation (#1929)

* Metrics interface

* Prometheus implementation

* NoOp implementation

Co-authored-by: chris <chris@Profanity.local>

metrics/options.go

@@ -0,0 +1,65 @@
+package metrics
+
+var (
+	// The Prometheus metrics will be made available on this port:
+	defaultPrometheusListenAddress = ":9000"
+	// This is the endpoint where the Prometheus metrics will be made available ("/metrics" is the default with Prometheus):
+	defaultPath = "/metrics"
+	// timingObjectives is the default spread of stats we maintain for timings / histograms:
+	defaultTimingObjectives = map[float64]float64{0.0: 0, 0.5: 0.05, 0.75: 0.04, 0.90: 0.03, 0.95: 0.02, 0.98: 0.001, 1: 0}
+)
+
+// Option powers the configuration for metrics implementations:
+type Option func(*Options)
+
+// Options for metrics implementations:
+type Options struct {
+	Address          string
+	Path             string
+	DefaultTags      Tags
+	TimingObjectives map[float64]float64
+}
+
+// NewOptions prepares a set of options:
+func NewOptions(opt ...Option) Options {
+	opts := Options{
+		Address:          defaultPrometheusListenAddress,
+		DefaultTags:      make(Tags),
+		Path:             defaultPath,
+		TimingObjectives: defaultTimingObjectives,
+	}
+
+	for _, o := range opt {
+		o(&opts)
+	}
+
+	return opts
+}
+
+// Path used to serve metrics over HTTP:
+func Path(value string) Option {
+	return func(o *Options) {
+		o.Path = value
+	}
+}
+
+// Address is the listen address to serve metrics on:
+func Address(value string) Option {
+	return func(o *Options) {
+		o.Address = value
+	}
+}
+
+// DefaultTags will be added to every metric:
+func DefaultTags(value Tags) Option {
+	return func(o *Options) {
+		o.DefaultTags = value
+	}
+}
+
+// TimingObjectives defines the desired spread of statistics for histogram / timing metrics:
+func TimingObjectives(value map[float64]float64) Option {
+	return func(o *Options) {
+		o.TimingObjectives = value
+	}
+}