[EuiButtonGroup] Add title when in iconOnly mode

CHANGELOG.md

@@ -1,5 +1,6 @@
 ## [`master`](https://github.com/elastic/eui/tree/master)
 
+- Added `title` in `EuiButtonGroup` when in `iconOnly` mode ([#5199](https://github.com/elastic/eui/pull/5199))
 - Updated `barSeriesStyle.displayValue` of the elastic-charts `Theme` for better default styles ([#4845](https://github.com/elastic/eui/pull/4845))
 
 **Bug fixes**

src/components/button/button_group/__snapshots__/button_group.test.tsx.snap

@@ -2316,6 +2316,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for multi 1`] = `
       class="euiButtonGroupButton euiButtonGroupButton--text euiButtonGroupButton--small euiButtonGroupButton-isIconOnly testClass1 testClass2"
       data-test-subj="test subject string"
       id="generated-id"
+      title="Option one"
       type="button"
     >
       <span
@@ -2338,6 +2339,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for multi 1`] = `
       class="euiButtonGroupButton euiButtonGroupButton--text euiButtonGroupButton--small euiButtonGroupButton-isIconOnly"
       data-test-subj="button01"
       id="generated-id"
+      title="Option two"
       type="button"
     >
       <span
@@ -2361,6 +2363,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for multi 1`] = `
       data-test-subj="button02"
       disabled=""
       id="generated-id"
+      title="Option three"
       type="submit"
     >
       <span
@@ -2399,6 +2402,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for single 1`] = `
       class="euiButtonGroupButton euiButtonGroupButton--text euiButtonGroupButton--small euiButtonGroupButton-isIconOnly testClass1 testClass2"
       data-test-subj="test subject string"
       for="generated-id"
+      title="Option one"
     >
       <span
         class="euiButtonContent euiButton__content"
@@ -2425,6 +2429,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for single 1`] = `
     <label
       class="euiButtonGroupButton euiButtonGroupButton--text euiButtonGroupButton--small euiButtonGroupButton-isIconOnly"
       for="generated-id"
+      title="Option two"
     >
       <span
         class="euiButtonContent euiButton__content"
@@ -2454,6 +2459,7 @@ exports[`EuiButtonGroup button props isIconOnly is rendered for single 1`] = `
       data-test-subj="button02"
       disabled=""
       id="generated-id"
+      title="Option three"
       type="submit"
     >
       <span

src/components/button/button_group/button_group_button.tsx

@@ -129,6 +129,7 @@ export const EuiButtonGroupButton: FunctionComponent<Props> = ({
         'data-text': innerText,
         title: innerText,
       }}
+      title={isIconOnly ? label?.toString() : undefined}
       {...elementProps}
       {...rest}
     >