fix(discover) Fetch the first series instead of count (#15355)

Instead of always fetching count(id) on first load, select the first
series in the option list. I've also fixed some downstream typing
problems that didn't come up earlier as yAxisSelector is a jsx file.

Author: markstory

src/sentry/static/sentry/app/actionCreators/events.tsx

@@ -16,7 +16,7 @@ type Options = {
   includePrevious?: boolean;
   limit?: number;
   query?: string;
-  yAxis?: 'event_count' | 'user_count';
+  yAxis?: string;
   field?: string[];
   referenceEvent?: string;
 };

src/sentry/static/sentry/app/views/events/eventsChart.jsx

@@ -101,9 +101,17 @@ class EventsChart extends React.Component {
     yAxisOptions: PropTypes.array,
   };
 
-  state = {
-    yAxis: 'event_count',
-  };
+  constructor(props) {
+    super(props);
+    const value =
+      props.yAxisOptions && props.yAxisOptions.length
+        ? props.yAxisOptions[0].value
+        : undefined;
+
+    this.state = {
+      yAxis: value,
+    };
+  }
 
   handleYAxisChange = value => {
     this.setState({yAxis: value});

src/sentry/static/sentry/app/views/events/utils/eventsRequest.tsx

@@ -47,7 +47,7 @@ type EventsRequestPartialProps = {
   includeTransformedData?: boolean;
   loading?: boolean;
   showLoading?: boolean;
-  yAxis?: 'event_count' | 'user_count';
+  yAxis?: string;
   children: (renderProps: RenderProps) => React.ReactNode;
 };