docs: clarify opentelemetry plugin enablement

[kayx23]

Description

The OpenTelemetry documentation previously explained Plugin Metadata before telling readers that the Plugin is disabled by default. This could lead users to configure metadata without loading the Plugin.

This PR:

• moves the enablement section before Plugin Metadata configuration
• tells readers to preserve the complete plugins list when enabling OpenTelemetry
• removes the literal ellipsis list entry from the YAML example
• replaces a stale statement about default settings and tenant ID with the current Plugin Metadata behavior
• applies the same changes to the English and Chinese documentation

Related to #13925.

Validation

• npx --yes markdownlint-cli@0.25.0 docs/en/latest/plugins/opentelemetry.md docs/zh/latest/plugins/opentelemetry.md
• ./utils/fix-zh-doc-segment.py
• git diff --check

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes in this PR
• I have updated both English and Chinese documentation
• I have verified that this documentation-only change is backward compatible