Add histogram for RoundSocket::addEvents

Track this function and event size distribution.

modules/common/src/main/mon.scala

@@ -235,6 +235,8 @@ object mon {
           val lateClient = inc(s"round.history.$platform.versionCheck.lateClient")
         }
       }
+      val newEvents = rec("round.history.newEvents")
+
       object mobile extends PlatformHistory("mobile")
       object site extends PlatformHistory("site")
       def apply(isMobile: lila.common.IsMobile): PlatformHistory =

modules/round/src/main/RoundSocket.scala

@@ -287,6 +287,7 @@ private[round] final class RoundSocket(
   }
 
   def notify(events: Events): Unit = {
+    lila.mon.round.history.newEvents(events.size)
     val vevents = history addEvents events
     members.foreachValue { m =>
       batchMsgs(m, vevents) foreach m.push