Add support for timestamp metrics with seconds precision (#558)

* Add support for timestamp metrics with seconds precision

This is useful for exporting oplog metrics in order to calculate the
oplog window as the difference between mongodb_ss_oplog_latestOptime
and mongodb_ss_oplog_earliestOptime

* Fix test expectations

Author: trvrnrth

exporter/metrics.go

@@ -208,7 +208,9 @@ func asFloat64(value interface{}) (*float64, error) {
 		f = v
 	case primitive.DateTime:
 		f = float64(v)
-	case primitive.A, primitive.ObjectID, primitive.Timestamp, primitive.Binary, string, []uint8, time.Time:
+	case primitive.Timestamp:
+		f = float64(v.T)
+	case primitive.A, primitive.ObjectID, primitive.Binary, string, []uint8, time.Time:
 		return nil, nil
 	default:
 		return nil, errors.Wrapf(errCannotHandleType, "%T", v)

exporter/metrics_test.go

@@ -141,7 +141,7 @@ func TestMakeRawMetric(t *testing.T) {
 		{value: float32(1.23), wantVal: pointer.ToFloat64(float64(float32(1.23)))},
 		{value: float64(1.23), wantVal: pointer.ToFloat64(1.23)},
 		{value: primitive.A{}, wantVal: nil},
-		{value: primitive.Timestamp{}, wantVal: nil},
+		{value: primitive.Timestamp{T: 123, I: 456}, wantVal: pointer.ToFloat64(123)},
 		{value: "zapp", wantVal: nil},
 		{value: []byte{}, wantVal: nil},
 		{value: time.Date(2020, 6, 15, 0, 0, 0, 0, time.UTC), wantVal: nil},