ResourceLoader: Log when MAXAGE_RECOVER is detected

Send a message to Logstash when this condition is reached.
It is expected that this happens for a portion of clients during the
5min after a deployment, such such use an informational message,
not a warning.

This level is already enabled in wmf-config for channel=resourceloader.

I'm adding this now as I'd like to rule out or confirm whether the
our own CDN caching plays a rule in the corruptions reported at
T321394. If so, we can dig deeper there. If not, then we're likely
looking at a problem relating to browser extensions, Wikipedia mirrors,
or user scripts; rather than something under our own control.

Bug: T321394
Change-Id: Iad2f5189da33551b59653c2a6783419d6ad955d0

includes/ResourceLoader/ResourceLoader.php

@@ -891,6 +891,7 @@ class ResourceLoader implements LoggerAwareInterface {
 			// to basically just debounce CDN traffic. This applies to:
 			// - Internal errors, e.g. due to misconfiguration.
 			// - Version mismatch, e.g. due to deployment race (T117587, T47877).
+			$this->logger->info( 'Client and server registry version out of sync' );
 			$maxage = self::MAXAGE_RECOVER;
 		} elseif ( $context->getVersion() === null ) {
 			// Resources that can't set a version, should have their updates propagate to