fix: use duration of handshake if no previous roundTripTime exists

The default `roundTripTime` of a `ServerDescription` is -1, which
means if that value is used we can potentially calculate a negative
`roundTripTime`. Instead, if no previous `roundTripTime` exists, we
use the duration of the initial handshake

NODE-2652

Author: mbroadst

lib/core/sdam/server.js

@@ -415,6 +415,10 @@ Object.defineProperty(Server.prototype, 'clusterTime', {
 });
 
 function calculateRoundTripTime(oldRtt, duration) {
+  if (oldRtt === -1) {
+    return duration;
+  }
+
   const alpha = 0.2;
   return alpha * duration + (1 - alpha) * oldRtt;
 }

lib/utils.js

@@ -741,7 +741,6 @@ function now() {
 
 function calculateDurationInMs(started) {
   if (typeof started !== 'number') {
-    console.trace('HERE');
     throw TypeError('numeric value required to calculate duration');
   }
 

test/unit/sdam/monitoring.test.js

@@ -87,20 +87,17 @@ describe('monitoring', function() {
     const topology = new Topology(mockServer.uri());
     topology.connect(err => {
       expect(err).to.not.exist;
+      expect(topology)
+        .property('description')
+        .property('servers')
+        .to.have.length(1);
 
-      setTimeout(() => {
-        expect(topology)
-          .property('description')
-          .property('servers')
-          .to.have.length(1);
-
-        const serverDescription = Array.from(topology.description.servers.values())[0];
-        expect(serverDescription)
-          .property('roundTripTime')
-          .to.be.greaterThan(0);
+      const serverDescription = Array.from(topology.description.servers.values())[0];
+      expect(serverDescription)
+        .property('roundTripTime')
+        .to.be.greaterThan(0);
 
-        topology.close(done);
-      }, 500);
+      topology.close(done);
     });
   });