Add optional statistics to API and web UI.

A new option (--web.statistics) enables the collection of some basic
information about requests and responses. This currently consists of
the most recent 10 requests that resulted in HTTP 4xx or 5xx errors.

types/types.go

@@ -228,3 +228,8 @@ type Digest struct {
 func CanonicalDomain(domain string) string {
 	return strings.ToLower(strings.TrimSpace(domain))
 }
+
+// Statistics provides options for monitoring request and response stats
+type Statistics struct {
+	RecentErrors int `description:"Number of recent errors logged"`
+}