influxdb: Check before assuming first column to be 'time'

When running queries like 'SHOW [...] CARDINALITY' introduced with
InfluxDB 1.4 there is only a single column 'count'. However, the data
source always assumed 'time' which threw an error because the code
couldn't find a non-time column to display.

This change actually checks this assumption and thereby fixes displaying
the result in a Singlestat in Table mode (closes #11476).

public/app/plugins/datasource/influxdb/influx_series.ts

@@ -151,11 +151,17 @@ export default class InfluxSeries {
 
     _.each(this.series, (series, seriesIndex) => {
       if (seriesIndex === 0) {
-        table.columns.push({ text: 'Time', type: 'time' });
+        j = 0;
+        // Check that the first column is indeed 'time'
+        if (series.columns[0] === 'time') {
+          // Push this now before the tags and with the right type
+          table.columns.push({ text: 'Time', type: 'time' });
+          j++;
+        }
         _.each(_.keys(series.tags), function(key) {
           table.columns.push({ text: key });
         });
-        for (j = 1; j < series.columns.length; j++) {
+        for (; j < series.columns.length; j++) {
           table.columns.push({ text: series.columns[j] });
         }
       }

public/app/plugins/datasource/influxdb/specs/influx_series.jest.ts

@@ -195,10 +195,34 @@ describe('when generating timeseries from influxdb response', function() {
 
       expect(table.type).toBe('table');
       expect(table.columns.length).toBe(5);
+      expect(table.columns[0].text).toEqual('Time');
       expect(table.rows[0]).toEqual([1431946625000, 'Africa', 'server2', 23, 10]);
     });
   });
 
+  describe('given table response from SHOW CARDINALITY', function() {
+    var options = {
+      alias: '',
+      series: [
+        {
+          name: 'cpu',
+          columns: ['count'],
+          values: [[37]],
+        },
+      ],
+    };
+
+    it('should return table', function() {
+      var series = new InfluxSeries(options);
+      var table = series.getTable();
+
+      expect(table.type).toBe('table');
+      expect(table.columns.length).toBe(1);
+      expect(table.columns[0].text).toEqual('count');
+      expect(table.rows[0]).toEqual([37]);
+    });
+  });
+
   describe('given annotation response', function() {
     describe('with empty tagsColumn', function() {
       var options = {