Influx: Adds start page for logs in Explore (#17521)

* Influx: Adds start page for logs in Explore
Closes #17499

* Influx: Rebased with master and removed conditional display of cheat sheet in explore
Cheat sheet now displays during both Metrics and Logs mode, though this is only
temporary.

public/app/plugins/datasource/influxdb/components/InfluxCheatSheet.tsx

+import React from 'react';
+
+const CHEAT_SHEET_ITEMS = [
+  {
+    title: 'Getting started',
+    label:
+      'Start by selecting a measurement and field from the dropdown above. You can then use the tag selector to further narrow your search.',
+  },
+];
+
+export default (props: any) => (
+  <div>
+    <h2>InfluxDB Cheat Sheet</h2>
+    {CHEAT_SHEET_ITEMS.map(item => (
+      <div className="cheat-sheet-item" key={item.title}>
+        <div className="cheat-sheet-item__title">{item.title}</div>
+        <div className="cheat-sheet-item__label">{item.label}</div>
+      </div>
+    ))}
+  </div>
+);

public/app/plugins/datasource/influxdb/components/InfluxStartPage.tsx

+import React, { PureComponent } from 'react';
+import { ExploreStartPageProps } from '@grafana/ui';
+import InfluxCheatSheet from './InfluxCheatSheet';
+
+export default class InfluxStartPage extends PureComponent<ExploreStartPageProps> {
+  render() {
+    return (
+      <div className="grafana-info-box grafana-info-box--max-lg">
+        <InfluxCheatSheet onClickExample={this.props.onClickExample} />
+      </div>
+    );
+  }
+}

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

 import InfluxDatasource from './datasource';
 import { InfluxQueryCtrl } from './query_ctrl';
 import { InfluxLogsQueryField } from './components/InfluxLogsQueryField';
+import InfluxStartPage from './components/InfluxStartPage';
+
 import {
   createChangeHandler,
   createResetHandler,
@@ -31,4 +33,5 @@ export const plugin = new DataSourcePlugin(InfluxDatasource)
   .setConfigCtrl(InfluxConfigCtrl)
   .setQueryCtrl(InfluxQueryCtrl)
   .setAnnotationQueryCtrl(InfluxAnnotationsQueryCtrl)
-  .setExploreLogsQueryField(InfluxLogsQueryField);
+  .setExploreLogsQueryField(InfluxLogsQueryField)
+  .setExploreStartPage(InfluxStartPage);