Proposal: use a `key` prop to override Meta tags

[DaniGuardiola]

What this changes

• <Meta> tags are not replaced by default.
• <Meta> tags are considered to be the same and overridden if the key (string) prop matches.

That's it. <Title> is still always replaced.

Why

• No complicated/surprising heuristics.
• Easier to maintain.
• Explicit and opt-in.
• Less surface for bugs.

Why not userland

• Server-side is a pain to get right, yet critical, especially for open-graph and other metadata (crawlers won't always execute JS).
• Duplicated logic - this library already needs to override <Title>, so userland de-duplication of meta tags would essentially add the same logic.
• I personally would expect my framework to handle this.
• Other popular frameworks like Next.js do exactly this. Fun fact, I only found out about this after writing this proposal. Other examples: Remix, Nuxt.

Status

Work in progress, need to figure out the tests but the basic implementation is done. Also, I think the key prop will currently be set as an attribute, so probably need to exclude it before rendering. Couldn't test yet because of local env issues.

Open questions

• Is key an appropriate/descriptive prop name for this?
• Should this behavior apply to other tags? (I think probably not)

Related

#34 #40