feat(gatsby): Gatsby Head API

[marvinjude]

Description

Link to RFC

Todo

• Warnings and descriptive error messages
• End-to-end tests - test(gatsby): Metadata management e2e tests #35979
• HMR for Head Export - feat(head-function-export): Hot update for Head export #36060
• Make Head export work in tsx pages - fix(head-function-export): Make tsx pages work #36114
• More e2e tests (tags cleanup, .tsx pages)

Documentation

#36125

[LekoArts]

Can you revert the image changes for contentful, e.g. packages/gatsby-source-contentful/src/__tests__/23df7612602d69edfc3eaf89c8c02248/347966-contentful-logo-wordmark-dark (1)-4cd185-original-1582664935 (1).png

[imjoshin]

[imjoshin]

Nit-picking: We have these assertions a lot, and adding/removing functionality in the future will be quite a task to update these. We could pull these assertions into a helper for this e2e suite only... Something like:

export const assertHeadElementAttributes({base, title, ...}) {
    cy.getTestElement(`base`)
    ...
}

Feel free to ignore. 😄

[marvinjude]

That'll be quite handy! 'style', 'link' and co above are test ids though not tag names

We could have something that asserts for text and attributes for a test id like so:

export const assertElementAttributesAndText = elements => {
  for (const { id, text, attributes } of elements) {
    if (text) cy.getTestElement(id).should("have.text", text)
    for (const [key, value] of Object.entries(attributes)) {
      cy.getTestElement(id).invoke("attr", key).should("equal", value)
    }
  }
}

const elements = [
  {
    id: "title",
    text: "Head function export should work",
    attributes: {
      content: "title",
    },
  },
]

assertElementAttributesAndText(elements)

Edit : Seem like an issue with all our tests though where we repeat things. Also when you want something like shoud.not then our abstraction may look messy.

We can add something that works for common cases in out test some other time via a non-metadata related PR.