fshttp: add prometheus metrics for HTTP status code

This patch adds rclone_http_status_code counter vector labeled by

* host,
* method,
* code.

It allows to see HTTP errors, backoffs etc.

The Metrics struct is designed for extensibility.
Adding new metrics is a matter of adding them to Metrics struct and including them in the response handling.

This feature has been discussed in the forum [1].

[1] https://forum.rclone.org/t/prometheus-metrics/14484

fs/rc/rcserver/rcserver.go

@@ -18,23 +18,22 @@ import (
 	"sync"
 	"time"
 
-	"github.com/rclone/rclone/fs/rc/webgui"
-
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
-	"github.com/skratchdot/open-golang/open"
-
 	"github.com/rclone/rclone/cmd/serve/httplib"
 	"github.com/rclone/rclone/fs"
 	"github.com/rclone/rclone/fs/accounting"
 	"github.com/rclone/rclone/fs/cache"
 	"github.com/rclone/rclone/fs/config"
+	"github.com/rclone/rclone/fs/fshttp"
 	"github.com/rclone/rclone/fs/list"
 	"github.com/rclone/rclone/fs/rc"
 	"github.com/rclone/rclone/fs/rc/jobs"
 	"github.com/rclone/rclone/fs/rc/rcflags"
+	"github.com/rclone/rclone/fs/rc/webgui"
 	"github.com/rclone/rclone/lib/http/serve"
 	"github.com/rclone/rclone/lib/random"
+	"github.com/skratchdot/open-golang/open"
 )
 
 var promHandler http.Handler
@@ -43,6 +42,13 @@ var onlyOnceWarningAllowOrigin sync.Once
 func init() {
 	rcloneCollector := accounting.NewRcloneCollector(context.Background())
 	prometheus.MustRegister(rcloneCollector)
+
+	m := fshttp.NewMetrics("rclone")
+	for _, c := range m.Collectors() {
+		prometheus.MustRegister(c)
+	}
+	fshttp.DefaultMetrics = m
+
 	promHandler = promhttp.Handler()
 }