Add logging transport to log HTTP requests in log level -3

The log level -3 is the minimum log level that is supported today, smaller than debug(-1) and -2(used to log some HRA related logs).

This commit adds a logging HTTP transport to log HTTP requests and responses to that log level.

It implements http.RoundTripper so that it can log each HTTP request with useful metadata like `from_cache` and `ratelimit_remaining`.
The former is set to `true` only when the logged request's response was served from ARC's in-memory cache.
The latter is set to X-RateLimit-Remaining response header value if and only if the response was served by GitHub, not by ARC's cache.

logging/transport.go

@@ -0,0 +1,51 @@
+// Package logging provides various logging helpers for ARC
+package logging
+
+import (
+	"net/http"
+
+	"github.com/go-logr/logr"
+	"github.com/gregjones/httpcache"
+)
+
+const (
+	// https://docs.github.com/en/rest/overview/resources-in-the-rest-api#rate-limiting
+	headerRateLimitRemaining = "X-RateLimit-Remaining"
+)
+
+// Transport wraps a transport with metrics monitoring
+type Transport struct {
+	Transport http.RoundTripper
+
+	Log *logr.Logger
+}
+
+func (t Transport) RoundTrip(req *http.Request) (*http.Response, error) {
+	resp, err := t.Transport.RoundTrip(req)
+	if resp != nil {
+		t.log(req, resp)
+	}
+	return resp, err
+}
+
+func (t Transport) log(req *http.Request, resp *http.Response) {
+	if t.Log == nil {
+		return
+	}
+
+	var args []interface{}
+
+	marked := resp.Header.Get(httpcache.XFromCache) == "1"
+
+	args = append(args, "from_cache", marked, "method", req.Method, "url", req.URL.String())
+
+	if !marked {
+		// Do not log outdated rate limit remaining value
+
+		remaining := resp.Header.Get(headerRateLimitRemaining)
+
+		args = append(args, "ratelimit_remaining", remaining)
+	}
+
+	t.Log.V(3).Info("Seen HTTP response", args...)
+}