From d4e3679e76283b97d336679af0194e8348a47668 Mon Sep 17 00:00:00 2001
From: "Stephen McQuay (smcquay)" <stephen@mcquay.me>
Date: Thu, 8 Mar 2018 13:50:12 -0800
Subject: [PATCH] Adds implementation

---
 metrics.go | 101 +++++++++++++++++++++++++++++++++++++++++++++++++++++
 resp.go    |  29 +++++++++++++++
 2 files changed, 130 insertions(+)
 create mode 100644 metrics.go
 create mode 100644 resp.go

diff --git a/metrics.go b/metrics.go
new file mode 100644
index 0000000..b1d667c
--- /dev/null
+++ b/metrics.go
@@ -0,0 +1,101 @@
+package metrics
+
+import (
+	"fmt"
+	"net/http"
+	"time"
+
+	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics provides a simple way to track latency and http status
+type Metrics struct {
+	latency *prometheus.SummaryVec
+	status  *prometheus.CounterVec
+}
+
+func New(prefix string) (*Metrics, error) {
+	m := &Metrics{
+		latency: prometheus.NewSummaryVec(prometheus.SummaryOpts{
+			Namespace: prefix,
+			Subsystem: "http",
+			Name:      "request_latency_ms",
+			Help:      "Latency in ms of http requests grouped by req path",
+		}, []string{"path"}),
+
+		status: prometheus.NewCounterVec(prometheus.CounterOpts{
+			Namespace: prefix,
+			Subsystem: "http",
+			Name:      "status_count",
+			Help:      "The count of http responses issued classified by status and api endpoint",
+		}, []string{"path", "code"}),
+	}
+	if err := m.registerPromMetrics(); err != nil {
+		return nil, errors.Wrap(err, "registration")
+	}
+	return m, nil
+}
+
+// registerPromMetrics registers all the metrics that eventmanger uses.
+func (m *Metrics) registerPromMetrics() error {
+	if err := prometheus.Register(m.latency); err != nil {
+		return errors.Wrap(err, "http request latency")
+	}
+
+	if err := prometheus.Register(m.status); err != nil {
+		return errors.Wrap(err, "http response counter")
+	}
+
+	return nil
+}
+
+// Wrap calls a http.Handler and tracks status code and latency.
+func (m *Metrics) Wrap(prefix string, h http.Handler) http.HandlerFunc {
+	return m.WrapFunc(prefix, h.ServeHTTP)
+}
+
+// WrapFunc calls a http.Handler and tracks status code and latency.
+func (m *Metrics) WrapFunc(prefix string, h http.HandlerFunc) http.HandlerFunc {
+	return func(w http.ResponseWriter, req *http.Request) {
+		start := time.Now()
+		defer func() {
+			m.HTTPLatency(prefix, start)
+		}()
+
+		lw := NewStatusRecorder(w)
+		h(lw, req)
+
+		m.HTTPStatus(prefix, lw.Status())
+	}
+}
+
+// HTTPLatency records a request latency for a given url path.
+func (m *Metrics) HTTPLatency(path string, start time.Time) {
+	m.latency.WithLabelValues(path).Observe(msSince(start))
+}
+
+// HTTPStatus records counts of http statuses, bucketed according to status types.
+func (m *Metrics) HTTPStatus(path string, status int) {
+	m.status.WithLabelValues(path, fmt.Sprintf("%d", bucketHTTPStatus(status))).Inc()
+}
+
+// bucketHTTPStatus rounds down to the nearest hundred to facilitate categorizing http statuses.
+func bucketHTTPStatus(i int) int {
+	return i - i%100
+}
+
+// msSince returns milliseconds since start.
+func msSince(start time.Time) float64 {
+	return float64(time.Since(start)) / float64(time.Millisecond)
+}
+
+// buckets returns the default prometheus buckets scaled to milliseconds.
+func buckets() []float64 {
+	r := []float64{}
+
+	for _, v := range prometheus.DefBuckets {
+		r = append(r, v*float64(time.Second/time.Millisecond))
+	}
+	return r
+}
diff --git a/resp.go b/resp.go
new file mode 100644
index 0000000..371a242
--- /dev/null
+++ b/resp.go
@@ -0,0 +1,29 @@
+package metrics
+
+import (
+	"net/http"
+)
+
+// StatusRecorder is a simple http status recorder
+type StatusRecorder struct {
+	http.ResponseWriter
+
+	status int
+}
+
+// NewStatusRecorder returns an initialized StatusRecorder, with 200 as the
+// default status.
+func NewStatusRecorder(w http.ResponseWriter) *StatusRecorder {
+	return &StatusRecorder{ResponseWriter: w, status: http.StatusOK}
+}
+
+// Status returns the cached http status value.
+func (sr *StatusRecorder) Status() int {
+	return sr.status
+}
+
+// WriteHeader caches the status, then calls the underlying ResponseWriter.
+func (sr *StatusRecorder) WriteHeader(status int) {
+	sr.status = status
+	sr.ResponseWriter.WriteHeader(status)
+}