[ML] Add a throughput description in the Deployment stats table (#163481)

## Summary

Resolves #161828

Adds a tooltip for the Throughput column explaining units.

<img width="854" alt="image"
src="0de0a0ea-d3d0-4da4-8d35-faa530de8405">

### Checklist

- [x] Any text added follows [EUI's writing
guidelines](https://elastic.github.io/eui/#/guidelines/writing), uses
sentence case text and includes [i18n
support](https://github.com/elastic/kibana/blob/main/packages/kbn-i18n/README.md)
- [x] Any UI touched in this PR does not create any new axe failures
(run axe in browser:
[FF](https://addons.mozilla.org/en-US/firefox/addon/axe-devtools/),
[Chrome](https://chrome.google.com/webstore/detail/axe-web-accessibility-tes/lhdoppojpmngadmnindnejefpokejbdd?hl=en-US))
- [x] This renders correctly on smaller devices using a responsive
layout. (You can test this [in your
browser](https://www.browserstack.com/guide/responsive-testing-on-local-server))
- [x] This was checked for [cross-browser
compatibility](https://www.elastic.co/support/matrix#matrix_browsers)

x-pack/plugins/ml/public/application/memory_usage/nodes_overview/allocated_models.tsx

@@ -122,13 +122,27 @@ export const AllocatedModels: FC<AllocatedModelsProps> = ({
       },
     },
     {
-      field: 'node.throughput_last_minute',
-      name: i18n.translate(
-        'xpack.ml.trainedModels.nodesList.modelsList.throughputLastMinuteHeader',
-        {
-          defaultMessage: 'Throughput',
-        }
+      name: (
+        <EuiToolTip
+          content={i18n.translate(
+            'xpack.ml.trainedModels.nodesList.modelsList.throughputLastMinuteTooltip',
+            {
+              defaultMessage: 'The number of requests processed in the last 1 minute.',
+            }
+          )}
+        >
+          <span>
+            {i18n.translate(
+              'xpack.ml.trainedModels.nodesList.modelsList.throughputLastMinuteHeader',
+              {
+                defaultMessage: 'Throughput',
+              }
+            )}
+            <EuiIcon size="s" color="subdued" type="questionInCircle" className="eui-alignTop" />
+          </span>
+        </EuiToolTip>
       ),
+      field: 'node.throughput_last_minute',
       width: '100px',
       truncateText: false,
       'data-test-subj': 'mlAllocatedModelsTableThroughput',