Add a metric for unfinished QUIC handshake count

Summary: Add visibility into the number of handshakes. We are already getting some partial visibility unintentionally through SLB_downstream_idle_conn.sum, but that metric includes actual idle connections and I need an explicit metric to set up rate limiting thresholds correctly.

Reviewed By: dddmello

Differential Revision: D50760028

fbshipit-source-id: 5c9063f80f7863b0b1a3e4d4c94c7b2207596c1f

quic/server/QuicServerWorker.cpp

@@ -49,6 +49,9 @@ bool isValidConnIdLength(const quic::ConnectionId& connId) {
 namespace quic {
 
 std::atomic_int globalUnfinishedHandshakes{0};
+int QuicServerWorker::getUnfinishedHandshakeCount() {
+  return globalUnfinishedHandshakes.load(std::memory_order_relaxed);
+}
 
 QuicServerWorker::QuicServerWorker(
     std::shared_ptr<QuicServerWorker::WorkerCallback> callback,

quic/server/QuicServerWorker.h

@@ -41,6 +41,9 @@ class QuicServerWorker : public QuicAsyncUDPSocketWrapper::ReadCallback,
                          public folly::EventRecvmsgCallback,
                          public folly::EventRecvmsgMultishotCallback,
                          public QuicTimerCallback {
+ public:
+  static int getUnfinishedHandshakeCount();
+
  private:
   struct MsgHdr : public folly::EventRecvmsgCallback::MsgHdr {
     static auto constexpr kBuffSize = 1024;