[ML] Single Metric Viewer: Fix brush update on short recent timespans. (

#56125) (#56139)

Fixes an issue where the context chart brush would render incorrectly for short recent time spans (e.g. 'now-15min`). Adds a check whether to display the brush and hide it if context and focus chart have the same timespan.

x-pack/legacy/plugins/ml/public/application/timeseriesexplorer/components/timeseries_chart/timeseries_chart.js

@@ -432,6 +432,9 @@ const TimeseriesChartIntl = injectI18n(
       }
       focusLoadTo = Math.min(focusLoadTo, contextXMax);
 
+      const brushVisibility = focusLoadFrom !== contextXMin || focusLoadTo !== contextXMax;
+      this.setBrushVisibility(brushVisibility);
+
       if (focusLoadFrom !== contextXMin || focusLoadTo !== contextXMax) {
         this.setContextBrushExtent(new Date(focusLoadFrom), new Date(focusLoadTo), true);
         const newSelectedBounds = {