fix: add calculated duration to server as roundTripTime

Round trip time was calculated and reported for servers in the
unified topology, but not marked for consideration during server
selection.

NODE-2356

lib/core/sdam/monitoring.js

@@ -172,6 +172,9 @@ function monitorServer(server, options) {
           return callback(err, null);
         }
 
+        // save round trip time
+        server.description.roundTripTime = duration;
+
         const isMaster = result.result;
         server.emit(
           'serverHeartbeatSucceeded',

test/unit/sdam/monitoring.test.js

@@ -0,0 +1,44 @@
+'use strict';
+const mock = require('mongodb-mock-server');
+const Topology = require('../../../lib/core/sdam/topology').Topology;
+const expect = require('chai').expect;
+
+describe('monitoring', function() {
+  let server;
+
+  after(() => mock.cleanup());
+  beforeEach(function() {
+    return mock.createServer().then(_server => (server = _server));
+  });
+
+  it('should record roundTripTime', function(done) {
+    server.setMessageHandler(request => {
+      const doc = request.document;
+      if (doc.ismaster) {
+        request.reply(Object.assign({}, mock.DEFAULT_ISMASTER));
+      } else if (doc.endSessions) {
+        request.reply({ ok: 1 });
+      }
+    });
+
+    // set `heartbeatFrequencyMS` to 250ms to force a quick monitoring check, and wait 500ms to validate below
+    const topology = new Topology(server.uri(), { heartbeatFrequencyMS: 250 });
+    topology.connect(err => {
+      expect(err).to.not.exist;
+
+      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);
+
+        topology.close(done);
+      }, 500);
+    });
+  });
+});