Add warning about unstable metrics in Prometheus documentation (#34097)

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Will need to backport into the 25.2 docs branch

<img width="820" height="154" alt="Screenshot 2025-11-12 at 2 38 25 PM"
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### Motivation
Adding this disclaimer because these metrics are inherently unstable
https://materializeinc.slack.com/archives/C08ACQNGSQK/p1762900180147019

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### Checklist

- [ ] This PR has adequate test coverage / QA involvement has been duly
considered. ([trigger-ci for additional test/nightly
runs](https://trigger-ci.dev.materialize.com/))
- [ ] This PR has an associated up-to-date [design
doc](https://github.com/MaterializeInc/materialize/blob/main/doc/developer/design/README.md),
is a design doc
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or is sufficiently small to not require a design.
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I have pinged the relevant PM to schedule a changelog post.

Author: SangJunBak

doc/user/content/manage/monitor/self-managed/prometheus.md

@@ -8,6 +8,10 @@ menu:
     identifier: "grafana-prometheus-sm"
 ---
 
+{{< warning >}}
+The metrics scraped are unstable and may change across releases.
+{{< /warning >}}
+
 This guide walks you through the steps required to monitor the performance and
 overall health of your Materialize instance using Prometheus and Grafana.