GODRIVER-1647 fix averageRTT calculations (#426)

Author: kritiravind

x/mongo/driver/topology/server.go

@@ -539,6 +539,7 @@ func (s *Server) heartbeat(conn *connection) (description.Server, *connection) {
 func (s *Server) updateAverageRTT(delay time.Duration) time.Duration {
 	if !s.averageRTTSet {
 		s.averageRTT = delay
+		s.averageRTTSet = true
 	} else {
 		alpha := 0.2
 		s.averageRTT = time.Duration(alpha*float64(delay) + (1-alpha)*float64(s.averageRTT))

x/mongo/driver/topology/server_rtt_test.go

@@ -14,14 +14,15 @@ import (
 	"time"
 
 	"github.com/stretchr/testify/require"
-	"go.mongodb.org/mongo-driver/internal/testutil/helpers"
+	testhelpers "go.mongodb.org/mongo-driver/internal/testutil/helpers"
 )
 
 // Test case for all server selection rtt spec tests.
 func TestServerSelectionRTTSpec(t *testing.T) {
 
 	type testCase struct {
-		AvgRttMs  json.Number `json:"avg_rtt_ms"`
+		// AvgRttMs is either "NULL" or float
+		AvgRttMs  interface{} `json:"avg_rtt_ms"`
 		NewRttMs  float64     `json:"new_rtt_ms"`
 		NewAvgRtt float64     `json:"new_avg_rtt"`
 	}
@@ -44,11 +45,8 @@ func TestServerSelectionRTTSpec(t *testing.T) {
 				var server Server
 
 				if test.AvgRttMs != "NULL" {
-					avg, err := test.AvgRttMs.Float64()
-					require.NoError(t, err)
-
-					server.averageRTT = time.Duration(avg * float64(time.Millisecond))
-					server.averageRTTSet = true
+					// If not "NULL", then must be a number, so typecast to float64
+					server.updateAverageRTT(time.Duration(test.AvgRttMs.(float64) * float64(time.Millisecond)))
 				}
 
 				server.updateAverageRTT(time.Duration(test.NewRttMs * float64(time.Millisecond)))