feat(plugin-vercel-analytics): add new vercel analytics plugin

[OzakIOne]

Motivation

This plugin adds support for Vercel Web Analytics.

You should only use this plugin if you use Vercel to host Docusaurus (which is a good option).

You are only supposed to use this plugin at most once per Docusaurus site, so it is not allowed to use multiple instances of this plugin. We reject the usage of a custom plugin id.

Plugin documentation: https://deploy-preview-9687--docusaurus-2.netlify.app/docs/api/plugins/@docusaurus/plugin-vercel-analytics/

This plugin only exposes the mode and debug options.

Building your site with docusaurus build --dev will turn on development env and debug mode automatically.

Other options are not supported for now, but we'll consider adding support for them.

• beforeSend()
• track

Please let us know if you need them and we'll try to add support later.

Test Plan

Unit tests

The Docusaurus website does not use Vercel, so we cannot dogfood this on our own website.

However, maintainer @slorber uses it on his newsletter website ThisWeekInReact.com and it works fine:

Test links

https://deploy-preview-9687--docusaurus-2.netlify.app/

Related issues/PRs

[github-actions]

⚡️ Lighthouse report for the deploy preview of this PR

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/docs/installation	🟠 69	🟢 96	🟢 100	🟢 100	🟠 88	Report
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/blog	🟠 71	🟢 100	🟢 100	🟢 90	🟠 88	Report
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[slorber]

need plugin docs

[slorber]

Needed before merge:

• little refactors, see inline comments
• docs
• options validation + unit tests
• forbid usage of pluginId !== "default" with clear error message

[slorber]

That almost looks good to merge

[slorber]

I think undefined ("auto") makes more sense as a default

For example docusaurus build --dev builds the site with node env "development" and we probably want it to impact the vercel plugin env in that case.

See also:

• https://docusaurus.io/docs/next/cli#docusaurus-build-sitedir
• https://vercel.com/docs/analytics/package#mode

[OzakIOne]

I don't understand what you mean with undefined ("auto") but I changed the default value to auto which makes more sense

[slorber]

Also use undefined?

https://vercel.com/docs/analytics/package#debug

This option is automatically enabled if the NODE_ENV environment variable is available and either development or test.

Similarly I think using docusaurus build --dev should turn this plugin to debug mode

[OzakIOne]

Same, didn't understand the use of undefined however modified according to your comment