feat(gateway): Add metrics PDK functions for sandbox

[Icarus9913]

Description

Docs about updating pdk functions for sandbox, to be releases with Kong 3.16.

Refer to https://github.com/Kong/kong-ee/pull/20833

Preview Links

Checklist

• Tested how-to docs. If not, note why here.
• All pages contain metadata.
• Any new docs link to existing docs.
• All autogenerated instructions render correctly (API, decK, Konnect, Kong Manager).
• Style guide (capitalized gateway entities, placeholder URLs) implemented correctly.
• Every page has a description entry in frontmatter.
• Add new pages to the product documentation index (if applicable).

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