feat(gateway): Metrics PDK

[lena-larionova]

Description

Fixes #6787

For reviewers: the how-to is part of a series, but you only need a few pieces to test it:

1. Run through the first page of the series to set up the directory structure: https://deploy-preview-7138--kongdeveloper.netlify.app/custom-plugins/get-started/set-up-plugin-project/
2. Save the schema.lua from the third page: https://deploy-preview-7138--kongdeveloper.netlify.app/custom-plugins/get-started/add-plugin-configuration/
3. Then follow the guide in this PR: https://deploy-preview-7138--kongdeveloper.netlify.app/custom-plugins/get-started/add-metrics/
   For the image in the docker compose file, substitute image: kong/kong-gateway-dev:3.16.0.0-rc.2

Preview Links

https://deploy-preview-7138--kongdeveloper.netlify.app/custom-plugins/get-started/add-metrics/
https://deploy-preview-7138--kongdeveloper.netlify.app/custom-plugins/metrics-pdk/
https://deploy-preview-7138--kongdeveloper.netlify.app/plugins/opentelemetry/

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