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/fshttp/prometheus.go

@@ -0,0 +1,51 @@
+package fshttp
+
+import (
+	"fmt"
+	"net/http"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+// Metrics provide Transport HTTP level metrics.
+type Metrics struct {
+	StatusCode *prometheus.CounterVec
+}
+
+// NewMetrics creates a new metrics instance, the instance shall be assigned to
+// DefaultMetrics before any processing takes place.
+func NewMetrics(namespace string) *Metrics {
+	return &Metrics{
+		StatusCode: prometheus.NewCounterVec(prometheus.CounterOpts{
+			Namespace: namespace,
+			Subsystem: "http",
+			Name:      "status_code",
+		}, []string{"host", "method", "code"}),
+	}
+}
+
+// DefaultMetrics specifies metrics used for new Transports.
+var DefaultMetrics = (*Metrics)(nil)
+
+// Collectors returns all prometheus metrics as collectors for registration.
+func (m *Metrics) Collectors() []prometheus.Collector {
+	if m == nil {
+		return nil
+	}
+	return []prometheus.Collector{
+		m.StatusCode,
+	}
+}
+
+func (m *Metrics) onResponse(req *http.Request, resp *http.Response) {
+	if m == nil {
+		return
+	}
+
+	var statusCode = 0
+	if resp != nil {
+		statusCode = resp.StatusCode
+	}
+
+	m.StatusCode.WithLabelValues(req.Host, req.Method, fmt.Sprint(statusCode)).Inc()
+}