Add a histogram to track how many server configs has been persisted

net/http/http_server_properties_manager.cc

Index: net/http/http_server_properties_manager.cc
diff --git a/net/http/http_server_properties_manager.cc b/net/http/http_server_properties_manager.cc
index 4130a57a4872d81799c523ba4d5507efce24e102..e467f567370f22e55c1771ec606ed7811fc56d42 100644
--- a/net/http/http_server_properties_manager.cc
+++ b/net/http/http_server_properties_manager.cc
@@ -775,6 +775,9 @@ void HttpServerPropertiesManager::UpdateCacheFromPrefsOnNetworkThread(
 
   http_server_properties_impl_->SetServerNetworkStats(server_network_stats_map);
 
+  UMA_HISTOGRAM_COUNTS_1000("Net.CountOfQuicServerInfos",
+                            quic_server_info_map->size());
+
   http_server_properties_impl_->SetQuicServerInfoMap(quic_server_info_map);
 
   // Update the prefs with what we have read (delete all corrupted prefs).