Fix mongodb_connections metric for mongos instances

exporter/diagnostic_data_collector.go

@@ -108,6 +108,25 @@ func (d *diagnosticDataCollector) collect(ch chan<- prometheus.Metric) {
 			logger.Error("cannot decode getDiagnosticData", "error", err)
 		}
 
+		// For mongos, getDiagnosticData returns empty data, so fall back to serverStatus
+		if nodeType == typeMongos && len(m) == 0 {
+			logger.Debug("getDiagnosticData returned empty data for mongos, falling back to serverStatus")
+			serverStatusCmd := bson.D{{Key: "serverStatus", Value: "1"}}
+			serverStatusRes := client.Database("admin").RunCommand(d.ctx, serverStatusCmd)
+			if serverStatusRes.Err() != nil {
+				logger.Error("failed to run serverStatus command for mongos", "error", serverStatusRes.Err())
+			} else {
+				var serverStatusData bson.M
+				if err := serverStatusRes.Decode(&serverStatusData); err != nil {
+					logger.Error("cannot decode serverStatus for mongos", "error", err)
+				} else {
+					// Wrap serverStatus data in the expected structure for makeMetrics
+					m = bson.M{"serverStatus": serverStatusData}
+					logger.Debug("Successfully retrieved serverStatus data for mongos")
+				}
+			}
+		}
+
 		logger.Debug("getDiagnosticData result")
 		debugResult(logger, m)
 

exporter/diagnostic_data_collector_test.go

@@ -254,6 +254,43 @@
 	}
 }
 
+func TestMongosDiagnosticDataCollectorMetrics(t *testing.T) {
+	ctx, cancel := context.WithTimeout(context.Background(), 3*time.Second)
+	defer cancel()
+
+	client := tu.DefaultTestClientMongoS(ctx, t)
+
+	logLevel := promslog.NewLevel()
+	err := logLevel.Set("debug")
+	require.NoError(t, err)
+	logger := promslog.New(&promslog.Config{
+		Level: logLevel,
+	})
+	ti := newTopologyInfo(ctx, client, logger)
+
+	dbBuildInfo, err := retrieveMongoDBBuildInfo(ctx, client, logger.With("component", "test"))
+	require.NoError(t, err)
+
+	c := newDiagnosticDataCollector(ctx, client, logger, true, ti, dbBuildInfo)
+
+	reg := prometheus.NewRegistry()
+	err = reg.Register(c)
+	require.NoError(t, err)
+	metrics := helpers.CollectMetrics(c)
+	actualMetrics := helpers.ReadMetrics(metrics)
+	filters := []string{
+		"mongodb_ss_connections",
+	}
+	actualMetrics = filterMetrics(actualMetrics, filters)
+	actualLines := helpers.Format(helpers.WriteMetrics(actualMetrics))
+	metricNames := getMetricNames(actualLines)
+
+	sort.Strings(filters)
+	for _, want := range filters {
+		assert.True(t, metricNames[want], fmt.Sprintf("missing %q metric", want))
+	}
+}
+
 // errorCountHandler is a custom handler that keeps tracks of the number of errors and warnings that were logged.
 // it discards all errors of other levels.
 type errorCountHandler struct {