change the Accept header sent to the exporters

Create a new version of the API endpoint (`/v2`).
This new version change the `Accept` HTTP header sent to the exporters requested
(from `application/openmetrics-text` … to `text/plain`) because the Go library
we use does not seems to support all OpenMetrics specificities ^1.

The requests sent to the exporters contains a new HTTP header `X-Promfetcher-API-version` with the Promfetchers API version.

^1: Java actuator does respect the OpenMetrics specifications; the double quotes in the comments is not supported for example:
> # HELP process_cpu_usage The \"recent cpu usage\" for the Java Virtual Machine process

See also:
[1]: prometheus/client_golang#829 (comment)
[2]: prometheus/common#214

api/init.go

@@ -22,20 +22,37 @@ func Register(rtr *mux.Router, metFetcher *fetchers.MetricsFetcher, broker *Brok
 	}
 
 	handlerMetrics := handlers.CompressHandler(http.HandlerFunc(api.metrics))
-	rtr.Handle("/v1/apps/{appIdOrPathOrName:.*}/metrics", handlerMetrics).
+	handlerOnlyAppMetrics := handlers.CompressHandler(forceOnlyForApp(http.HandlerFunc(api.metrics)))
+
+	// API v1: deprecated
+	routerApiV1 := rtr.PathPrefix("/v1").Subrouter()
+	routerApiV1.Handle("/apps/{appIdOrPathOrName:.*}/metrics", handlerMetrics).
 		Methods(http.MethodGet)
 
-	rtr.Handle("/v1/apps/metrics", handlerMetrics).
+	routerApiV1.Handle("/apps/metrics", handlerMetrics).
 		Methods(http.MethodGet)
 
-	handlerOnlyAppMetrics := handlers.CompressHandler(forceOnlyForApp(http.HandlerFunc(api.metrics)))
+	routerApiV1.Handle("/apps/{appIdOrPathOrName:.*}/only-app-metrics", handlerOnlyAppMetrics).
+		Methods(http.MethodGet)
+
+	routerApiV1.Handle("/apps/only-app-metrics", handlerOnlyAppMetrics).
+		Methods(http.MethodGet)
+
+	// API v2
+	routerApiV2 := rtr.PathPrefix("/v2").Subrouter()
+	routerApiV2.Handle("/apps/{appIdOrPathOrName:.*}/metrics", handlerMetrics).
+		Methods(http.MethodGet)
+
+	routerApiV2.Handle("/apps/metrics", handlerMetrics).
+		Methods(http.MethodGet)
 
-	rtr.Handle("/v1/apps/{appIdOrPathOrName:.*}/only-app-metrics", handlerOnlyAppMetrics).
+	routerApiV2.Handle("/apps/{appIdOrPathOrName:.*}/only-app-metrics", handlerOnlyAppMetrics).
 		Methods(http.MethodGet)
 
-	rtr.Handle("/v1/apps/only-app-metrics", handlerOnlyAppMetrics).
+	routerApiV2.Handle("/apps/only-app-metrics", handlerOnlyAppMetrics).
 		Methods(http.MethodGet)
 
+	// non-API routes
 	rtr.NewRoute().MatcherFunc(func(req *http.Request, m *mux.RouteMatch) bool {
 		return strings.HasPrefix(req.URL.Path, "/broker/v2")
 	}).Handler(http.StripPrefix("/broker", broker.Handler()))

api/metrics.go

@@ -3,15 +3,27 @@ package api
 import (
 	"fmt"
 	"net/http"
+	"regexp"
 	"strings"
 
 	"github.com/gorilla/mux"
+	"github.com/orange-cloudfoundry/promfetcher/models"
 	"github.com/prometheus/common/expfmt"
 
 	"github.com/orange-cloudfoundry/promfetcher/errors"
 )
 
 func (a Api) metrics(w http.ResponseWriter, req *http.Request) {
+	// extract the API version from the requested path (ie: /v2)
+	// and set it to an HTTP header
+	apiVersion := regexp.MustCompile("/v([0-9]+)(?:/|$)").FindStringSubmatch(req.URL.Path)
+	if len(apiVersion) == 2 {
+		req.Header.Set(models.XPromfetcherApiVersion, apiVersion[1])
+	} else {
+		// default to v1
+		req.Header.Set(models.XPromfetcherApiVersion, "1")
+	}
+
 	appIdOrPathOrName, ok := mux.Vars(req)["appIdOrPathOrName"]
 	if !ok {
 		appIdOrPathOrName = req.URL.Query().Get("app")

models/api.go

@@ -0,0 +1,4 @@
+package models
+
+//XPromfetcherApiVersion name of the HTTP header corresponding to the Promfetcher API version
+const XPromfetcherApiVersion = `X-Promfetcher-API-version`

scrapers/scrape.go

@@ -12,10 +12,9 @@ import (
 	"github.com/orange-cloudfoundry/promfetcher/clients"
 	"github.com/orange-cloudfoundry/promfetcher/errors"
 	"github.com/orange-cloudfoundry/promfetcher/models"
+	"github.com/prometheus/common/expfmt"
 )
 
-const acceptHeader = `application/openmetrics-text; version=0.0.1,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`
-
 type Scraper struct {
 	backendFactory *clients.BackendFactory
 	db             *gorm.DB
@@ -69,7 +68,14 @@ func (s Scraper) Scrape(route *models.Route, metricPathDefault string, headers h
 			req.Header[k] = v
 		}
 	}
-	req.Header.Add("Accept", acceptHeader)
+	// Prometheus parser is not OpenMetrics compliant
+	// See: prometheus/common issues: 214, 829
+	req.Header.Set("Accept", string(expfmt.FmtText))
+	// keep the OpenMetrics accept HTTP header for the /v1 endpoint
+	if req.Header.Get(models.XPromfetcherApiVersion) == "1" {
+		req.Header.Set(models.XPromfetcherApiVersion, "1")
+		req.Header.Set("Accept", `application/openmetrics-text; version=0.0.1,text/plain;version=0.0.4;q=0.5,*/*;q=0.1`)
+	}
 	req.Header.Add("Accept-Encoding", "gzip")
 	req.Header.Set("X-Prometheus-Scrape-Timeout-Seconds", fmt.Sprintf("%f", (30*time.Second).Seconds()))
 	req.Header.Set("X-Forwarded-Proto", scheme)