Add security protocol trace event.

Reviewed By: siyengar

Differential Revision: D2805183

fb-gh-sync-id: 9ed320dd3762658e993daa658100d0c3c617d210

folly/io/async/AsyncSSLSocket.h

@@ -275,6 +275,8 @@ class AsyncSSLSocket : public virtual AsyncSocket {
   virtual bool connecting() const override;
   virtual std::string getApplicationProtocol() noexcept override;
 
+  virtual std::string getSecurityProtocol() const override { return "TLS"; }
+
   bool isEorTrackingEnabled() const override;
   virtual void setEorTracking(bool track) override;
   virtual size_t getRawBytesWritten() const override;

folly/io/async/AsyncTransport.h

@@ -573,6 +573,11 @@ class AsyncTransportWrapper : virtual public AsyncTransport,
   virtual std::string getApplicationProtocol() noexcept {
     return "";
   }
+
+  /**
+   * Returns the name of the security protocol being used.
+   */
+  virtual std::string getSecurityProtocol() const { return ""; }
 };
 
 } // folly