[UnifiedHistogram] Prevent double progress bar (#180456)

- Closes https://github.com/elastic/kibana/issues/180434

~~This PR removes the second progress bar. I think with all recent
changes it became redundant.~~

This PR adds a delay for UnifiedHistogram progress bar. It helps to
prevent double loading indicator in ES|QL mode.

src/plugins/unified_histogram/public/chart/chart.tsx

@@ -11,7 +11,7 @@ import type { Observable } from 'rxjs';
 import { Subject } from 'rxjs';
 import useObservable from 'react-use/lib/useObservable';
 import { IconButtonGroup, type IconButtonGroupProps } from '@kbn/shared-ux-button-toolbar';
-import { EuiFlexGroup, EuiFlexItem, EuiProgress } from '@elastic/eui';
+import { EuiFlexGroup, EuiFlexItem, EuiProgress, EuiDelayRender } from '@elastic/eui';
 import { i18n } from '@kbn/i18n';
 import type {
   EmbeddableComponentProps,
@@ -366,12 +366,15 @@ export function Chart({
             data-test-subj="unifiedHistogramRendered"
           >
             {isChartLoading && (
-              <EuiProgress
-                size="xs"
-                color="accent"
-                position="absolute"
-                data-test-subj="unifiedHistogramProgressBar"
-              />
+              /*
+                There are 2 different loaders which can appear above the chart. One is from the embeddable and one is from the UnifiedHistogram.
+                The idea is to show UnifiedHistogram loader until we get a new query params which would trigger the embeddable loader.
+                Updates to the time range can come earlier than the query updates which we delay on purpose for text based mode,
+                this is why it might get both loaders. We should find a way to resolve that better.
+              */
+              <EuiDelayRender delay={500} data-test-subj="unifiedHistogramProgressBar">
+                <EuiProgress size="xs" color="accent" position="absolute" />
+              </EuiDelayRender>
             )}
             <HistogramMemoized
               abortController={abortController}