docs: Add mattermost_system_server_info metric exposing version and build info

source/about/drafts/mattermost-pr-37209.md

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+# Add mattermost_system_server_info metric exposing version and build info
+
+> **Status:** Draft - auto-generated. Requires editorial review.
+> **Source:** [mattermost/mattermost#37209](https://github.com/mattermost/mattermost/pull/37209)
+> **Dev reviewer:** @lieut-data
+
+---
+
+=== CAPABILITY SUMMARY ===
+
+Capability change (one sentence): Administrators can now query Prometheus for the exact server version and build details (version, build number, build hash, enterprise build hash) running on each Mattermost instance using the new `mattermost_system_server_info` metric.
+
+PARITY or NET-NEW: NET-NEW
+
+Docs scope: Update existing
+
+Target personas: System Admin, IT Service Operations
+
+---
+
+=== DOCUMENTATION DRAFT ===
+
+**Recommended doc location:**
+- Mattermost Performance Monitoring / Metrics Reference page (the page documenting available Prometheus metrics)
+
+---
+
+**Proposed content (ready to paste):**
+
+#### Server info metric
+
+From Mattermost v11.9.0, a new Prometheus info metric is available:
+
+```
+mattermost_system_server_info
+```
+
+This gauge is always set to `1` and exposes the running server's version and build details as labels. It is registered at server startup.
+
+**Labels:**
+
+| Label | Description |
+|---|---|
+| `version` | Current Mattermost server version |
+| `build_number` | Build number |
+| `build_hash` | Git commit hash of the server build |
+| `build_hash_enterprise` | Git commit hash of the enterprise build |
+
+On cloud deployments, the metric also includes the existing cloud `ConstLabels` (installation ID, group, database cluster).
+
+**Example query:**
+
+To confirm which version is running across all servers in an environment:
+
+```promql
+count by (version) (mattermost_system_server_info)
+```
+
+This is useful for verifying that a deployment or cherry-pick has fully rolled out.
+
+---
+
+**Notes:**
+- This follows the standard [Prometheus info metric pattern](https://www.robustperception.io/how-to-have-labels-for-machine-roles-in-prometheus/) (fixed gauge value of `1`, data carried in labels).
+- The metric is registered once at startup; no ongoing collection cost.
+- milestone.title: `v11.9.0` — used as version reference per PR metadata.