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.

traefik.sample.toml

@@ -271,6 +271,10 @@
 #
 # Optional
 # ReadOnly = false
+#
+# Enable more detailed statistics
+# [web.statistics]
+#   RecentErrors = 10
 
 # To enable basic auth on the webui
 # with 2 user/pass: test:test and test2:test2