[8.18] [Navigation] Add stack monitoring to Observability sidebar (#210140) (#211133)

# Backport

This will backport the following commits from `main` to `8.18`:
- [[Navigation] Add stack monitoring to Observability sidebar
(#210140)](https://github.com/elastic/kibana/pull/210140)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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Add stack monitoring to Observability sidebar (#210140)\n\n##
Summary\r\n\r\nThis PR adds `Stack Monitoring` to the side navigation.
\r\nAddresses:
https://github.com/elastic/kibana-team/issues/1312\r\n\r\n![Screenshot
2025-02-07 at 09
46\r\n34](https://github.com/user-attachments/assets/1a9f6d2e-4b13-427f-bbb1-1cf18a1ec86a)","sha":"72265c026b875dd6dcefb07481784afd19f9347d","branchLabelMapping":{"^v9.1.0$":"main","^v8.19.0$":"8.x","^v(\\d+).(\\d+).\\d+$":"$1.$2"}},"sourcePullRequest":{"labels":["enhancement","release_note:skip","v9.0.0","Team:obs-ux-management","backport:version","v8.18.0","v9.1.0","v8.19.0"],"title":"[Navigation]
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Add stack monitoring to Observability sidebar (#210140)\n\n##
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Add stack monitoring to Observability sidebar (#210140)\n\n##
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\r\nAddresses:
https://github.com/elastic/kibana-team/issues/1312\r\n\r\n![Screenshot
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BACKPORT-->

Co-authored-by: Krzysztof Kowalczyk <krzysztof.kowalczyk@elastic.co>

x-pack/solutions/observability/plugins/observability/public/navigation_tree.ts

@@ -535,6 +535,10 @@ function createNavTree({ streamsAvailable }: { streamsAvailable?: boolean }) {
               },
             ],
           },
+          {
+            id: 'monitoring',
+            link: 'monitoring',
+          },
           {
             link: 'integrations',
           },