Add num_suspect_servers stat

Summary: add current number of "suspect" servers to stats.

Test Plan: mcrouter unit tests

Reviewed By: @alikhtarov

Differential Revision: D2049957

mcrouter/TkoTracker.cpp

@@ -217,6 +217,19 @@ TkoTrackerMap::getSuspectServers() {
   return result;
 }
 
+size_t TkoTrackerMap::getSuspectServersCount() {
+  size_t result = 0;
+  std::lock_guard<std::mutex> lock(mx_);
+  for (const auto& it : trackers_) {
+    if (auto tracker = it.second.lock()) {
+      if (tracker->consecutiveFailureCount() > 0) {
+        ++result;
+      }
+    }
+  }
+  return result;
+}
+
 std::weak_ptr<TkoTracker> TkoTrackerMap::getTracker(folly::StringPiece key) {
   std::lock_guard<std::mutex> lock(mx_);
   return folly::get_default(trackers_, key);

mcrouter/TkoTracker.h

@@ -199,7 +199,12 @@ class TkoTrackerMap {
                      const size_t maxSoftTkos);
 
   /**
-   * @return  servers that recently received error replies.
+   * @return  number of servers that recently returned error replies.
+   */
+  size_t getSuspectServersCount();
+
+  /**
+   * @return  servers that recently returned error replies.
    *   Format: {
    *     server ip => ( is server marked as TKO?, number of failures )
    *   }

mcrouter/stat_list.h

@@ -23,6 +23,7 @@
   STUI(num_servers_down, 0, 1)
   STUI(num_servers_closed, 0, 1)
   STUI(num_clients, 0, 1)
+  STUI(num_suspect_servers, 0, 1)
 #undef GROUP
 #define GROUP ods_stats | mcproxy_stats
   /* Total reqs in mc client yet to be sent to memcache. */

mcrouter/stats.cpp

@@ -334,6 +334,9 @@ void prepare_stats(McrouterInstance* router, stat_t* stats) {
     );
   }
 
+  stat_set_uint64(stats, num_suspect_servers_stat,
+                  router->tkoTrackerMap().getSuspectServersCount());
+
   stat_set_uint64(stats, mcc_txbuf_reqs_stat, destStats.pendingRequests);
   stat_set_uint64(stats, mcc_waiting_replies_stat, destStats.inflightRequests);