[MUL-1346] chore(studio): hide metrics export banner for HA projects (#49781)

Hides the "Export Metrics to your dashboards. Get started for free!"
banner (`ObservabilityLink`) for High Availability (Multigres) projects
— the Metrics API it links to is not available for them. The check lives
inside the shared component, so it applies to every observability
sub-page that renders the banner; non-HA projects are unchanged.

Addresses
[MUL-1346](https://linear.app/supabase/issue/MUL-1346/database-observability-dashboard-remove-text-for-unsupported-feature).

## To test

- On an HA project: open Observability → Database (and any other
observability sub-page, e.g. Auth) — the "Export Metrics to your
dashboards" banner at the bottom of the page should be gone
- On a non-HA project: same pages — the banner still shows, with "Get
started for free!" linking to the metrics docs

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->

## Summary by CodeRabbit

* **Bug Fixes**
* Metrics export links are now hidden for High Availability projects,
where the Metrics API isn’t available.
* Existing metrics export functionality remains unchanged for supported
projects.

<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Co-authored-by: Alaister Young <10985857+alaister@users.noreply.github.com>

apps/studio/components/ui/ObservabilityLink.tsx

@@ -1,8 +1,14 @@
 import Link from 'next/link'
 
+import { useHighAvailability } from '@/hooks/misc/useHighAvailability'
 import { DOCS_URL } from '@/lib/constants'
 
 export const ObservabilityLink = () => {
+  const { isHighAvailability } = useHighAvailability()
+
+  // The Metrics API is not available for High Availability (Multigres) projects
+  if (isHighAvailability) return null
+
   return (
     <div className="flex items-center justify-center gap-1.5 text-sm">
       <p className="text-foreground-light">