strip routing prefix in all cases if configured

Summary:
The routing prefix should always be stripped.
We discovered samples where the routing prefix is not stripped; after investigations these are apparently only TKOs with PoolRoute: https://fburl.com/scuba/mcrouter_requests/6tq7mwtp
This is due to inconsistent logging code in the DestinationRoute and this diff fixes it.

Reviewed By: stuclar

Differential Revision: D50704110

fbshipit-source-id: d0e0a22c4537f52048320c40f928957124f4ee05

mcrouter/routes/DestinationRoute.h

@@ -230,21 +230,29 @@ class DestinationRoute {
     if (bucketIdOptional.has_value()) {
       bucketId = *bucketIdOptional;
     }
+    std::optional<Request> newReq;
+    folly::StringPiece strippedRoutingPrefix;
+    if (!keepRoutingPrefix_ && !req.key_ref()->routingPrefix().empty()) {
+      newReq.emplace(req);
+      newReq->key_ref()->stripRoutingPrefix();
+      strippedRoutingPrefix = req.key_ref()->routingPrefix();
+    }
+    const auto& reqToSend = newReq ? *newReq : req;
     RpcStatsContext rpcContext;
     ctx.onBeforeRequestSent(
         poolName_,
         *destination_->accessPoint(),
-        folly::StringPiece(),
-        req,
+        strippedRoutingPrefix,
+        reqToSend,
         fiber_local<RouterInfo>::getRequestClass(),
         now,
         bucketId);
     ctx.onReplyReceived(
         poolName_,
         std::optional<size_t>(indexInPool_),
         *destination_->accessPoint(),
-        folly::StringPiece(),
-        req,
+        strippedRoutingPrefix,
+        reqToSend,
         reply,
         fiber_local<RouterInfo>::getRequestClass(),
         now,