[profiling] Ensure constant colors in profiling bar chart (#146696)

## Summary

Make sure that identical objects in the bar chart always get assigned
the same colors.

Co-authored-by: kibanamachine <42973632+kibanamachine@users.noreply.github.com>
Co-authored-by: Tim Rühsen <tim.ruhsen@elastic.co>
(cherry picked from commit 0a065f3)

x-pack/plugins/profiling/common/topn.ts

@@ -255,10 +255,22 @@ export function groupSamplesByCategory({
     rotations: Math.ceil(subcharts.length / 10),
   });
 
+  // We want the mapping from the category string to the color to be constant,
+  // so that the same category string will always map to the same color.
+  const stringhash = (s: string): number => {
+    let hash: number = 0;
+    for (let i = 0; i < s.length; i++) {
+      const ch = s.charCodeAt(i);
+      hash = (hash << 5) - hash + ch; // eslint-disable-line no-bitwise
+      hash &= hash; // eslint-disable-line no-bitwise
+    }
+    return hash % subcharts.length;
+  };
+
   return orderBy(subcharts, ['Percentage', 'Category'], ['desc', 'asc']).map((chart, index) => {
     return {
       ...chart,
-      Color: colors[index],
+      Color: colors[stringhash(chart.Category)],
       Index: index + 1,
       Series: chart.Series.map((value) => {
         return {