docs(otel-collector): add v3.1 upgrade guide

charts/otel-collector-lerian/docs/UPGRADE-3.1.md

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+# Helm Upgrade from v3.0.x to v3.1.x
+## Topics
+- **[Overview](#overview)**- **[Version changes](#version-changes)**- **[Configuration changes](#configuration-changes)**- **[Template changes](#template-changes)**- **[Migration steps](#migration-steps)**- **[Preview changes before upgrading](#preview-changes-before-upgrading)**- **[Command to upgrade](#command-to-upgrade)**
+
+## Overview
+This guide covers the `otel-collector-lerian` chart upgrade from `3.0.0` to `3.1.0-beta.2`. It was generated retroactively from the chart history and focuses on minor version changes; patch-only releases are intentionally ignored.
+
+Because this is a minor upgrade, the expected path is an in-place Helm upgrade after reviewing new values and changed defaults.
+
+## Version changes
+
+| Field | Previous | Current |
+|-------|----------|---------|
+| Chart version | `3.0.0` | `3.1.0-beta.2` |
+| App version | `0.1.0` | `0.1.0` |
+
+## Configuration changes
+
+### Added values
+
+_No direct values.yaml key changes detected._
+
+### Removed values
+
+_No direct values.yaml key changes detected._
+
+### Changed operational values
+
+_No image, env, secret, probe, ingress, service, port, or enablement changes detected in values.yaml._
+
+## Template changes
+
+### Added files
+
+- No chart files added.
+
+### Removed files
+
+- No chart files removed.
+
+### Modified files
+
+- `charts/otel-collector-lerian/CHANGELOG.md`
+- `charts/otel-collector-lerian/Chart.yaml`
+
+## Migration steps
+
+1. Read this guide and compare your custom values against `charts/otel-collector-lerian/values.yaml`.
+2. Add any required new values for your environment, especially secrets, configmaps, probes, ingress, and service settings.
+3. Render the chart locally with your production values and review the manifest diff.
+4. Apply the upgrade in a controlled environment before production.
+
+## Preview changes before upgrading
+
+```bash
+helm diff upgrade otel-collector-lerian ./charts/otel-collector-lerian \
+  --namespace <namespace> \
+  --values <your-values.yaml>
+```
+
+## Command to upgrade
+
+```bash
+helm upgrade otel-collector-lerian ./charts/otel-collector-lerian \
+  --namespace <namespace> \
+  --values <your-values.yaml>
+```