Don't set Site URL on requests from ELB HealthChecker (#12996)

* Fix for AWS ELB issue

When Metabase is deployed to AWS behind ELB the very first call to it is made by ELB health check. Out of `origin`, `x-forwarded-host`,  `host` in this case only `host` is populated and it is populated with internal private IP. This causes `site-url` to be populated with URL with private IP in it. The fix is checks for caller user agent and if it contains "HealthChecker" - does not set `site-url` at this time

* Added missing user-agent

* Added check for nil user-agent

src/metabase/middleware/misc.clj

@@ -40,10 +40,10 @@
 ;;
 ;; Effectively the very first API request that gets sent to us (usually some sort of setup request) ends up setting
 ;; the (initial) value of `site-url`
-(defn- maybe-set-site-url* [{{:strs [origin x-forwarded-host host] :as headers} :headers, :as request}]
+(defn- maybe-set-site-url* [{{:strs [origin x-forwarded-host host user-agent] :as headers} :headers, :as request}]
   (when (and (mdb/db-is-setup?)
              (not (public-settings/site-url))
-             api/*current-user*)
+              api/*current-user* (or (nil? user-agent) ((complement clojure.string/includes?) user-agent "HealthChecker"))); Not setting URL if it's a healthcheck by ELB
     (when-let [site-url (or origin x-forwarded-host host)]
       (log/info (trs "Setting Metabase site URL to {0}" site-url))
       (try