rename prometheus metrics and vars

Make names a little more consistent and align with naming docs at
https://prometheus.io/docs/practices/naming/

imageproxy.go

@@ -150,7 +150,7 @@ func (p *Proxy) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		h = tphttp.TimeoutHandler(h, p.Timeout, "Gateway timeout waiting for remote resource.")
 	}
 
-	timer := prometheus.NewTimer(httpRequestsResponseTime)
+	timer := prometheus.NewTimer(metricRequestDuration)
 	defer timer.ObserveDuration()
 	h.ServeHTTP(w, r)
 }
@@ -191,7 +191,7 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 		msg := fmt.Sprintf("error fetching remote image: %v", err)
 		p.log(msg)
 		http.Error(w, msg, http.StatusInternalServerError)
-		remoteImageFetchErrors.Inc()
+		metricRemoteErrors.Inc()
 		return
 	}
 	// close the original resp.Body, even if we wrap it in a NopCloser below
@@ -203,7 +203,7 @@ func (p *Proxy) serveImage(w http.ResponseWriter, r *http.Request) {
 	}
 
 	if cached {
-		requestServedFromCacheCount.Inc()
+		metricServedFromCache.Inc()
 	}
 
 	copyHeader(w.Header(), resp.Header, "Cache-Control", "Last-Modified", "Expires", "Etag", "Link")

metrics.go

@@ -5,29 +5,32 @@ import (
 )
 
 var (
-	requestServedFromCacheCount = prometheus.NewCounter(
+	metricServedFromCache = prometheus.NewCounter(
 		prometheus.CounterOpts{
-			Name: "requests_served_from_cache",
-			Help: "Number of requests served from cache.",
+			Namespace: "imageproxy",
+			Name:      "requests_served_from_cache_total",
+			Help:      "Number of requests served from cache.",
 		})
-	imageTransformationSummary = prometheus.NewSummary(prometheus.SummaryOpts{
-		Name: "image_transformation_seconds",
-		Help: "Time taken for image transformations in seconds.",
+	metricTransformationDuration = prometheus.NewSummary(prometheus.SummaryOpts{
+		Namespace: "imageproxy",
+		Name:      "transformation_duration_seconds",
+		Help:      "Time taken for image transformations in seconds.",
 	})
-	remoteImageFetchErrors = prometheus.NewCounter(prometheus.CounterOpts{
-		Name: "remote_image_fetch_errors",
-		Help: "Total image fetch failures",
+	metricRemoteErrors = prometheus.NewCounter(prometheus.CounterOpts{
+		Namespace: "imageproxy",
+		Name:      "remote_fetch_errors_total",
+		Help:      "Total remote image fetch errors",
 	})
-	httpRequestsResponseTime = prometheus.NewSummary(prometheus.SummaryOpts{
+	metricRequestDuration = prometheus.NewSummary(prometheus.SummaryOpts{
 		Namespace: "http",
-		Name:      "response_time_seconds",
+		Name:      "request_duration_seconds",
 		Help:      "Request response times",
 	})
 )
 
 func init() {
-	prometheus.MustRegister(imageTransformationSummary)
-	prometheus.MustRegister(requestServedFromCacheCount)
-	prometheus.MustRegister(remoteImageFetchErrors)
-	prometheus.MustRegister(httpRequestsResponseTime)
+	prometheus.MustRegister(metricTransformationDuration)
+	prometheus.MustRegister(metricServedFromCache)
+	prometheus.MustRegister(metricRemoteErrors)
+	prometheus.MustRegister(metricRequestDuration)
 }

transform.go

@@ -275,7 +275,7 @@ func exifOrientation(r io.Reader) (opt Options) {
 // transformImage modifies the image m based on the transformations specified
 // in opt.
 func transformImage(m image.Image, opt Options) image.Image {
-	timer := prometheus.NewTimer(imageTransformationSummary)
+	timer := prometheus.NewTimer(metricTransformationDuration)
 	defer timer.ObserveDuration()
 
 	// Parse crop and resize parameters before applying any transforms.