CORE: Allow Setting Index Autocreate to _template

[original-brownbear]

• Enables setting autocreate by template as described in Replace action.auto_create_index with a template setting #20640
• Added default template called _default
  • Chose the underscore name that wasn't legal before to not have any collisions with existing templates for sure
  • Gave it the highest possible order so it doesn't override any other matching templates
• Setting the old global setting to _template enables the new functionality
• Closes Replace action.auto_create_index with a template setting #20640

---

• Marked WIP because I have some open questions here, that I will ask below in a bit.
• Also, solution for watcher compatibility is missing yet, incoming though

[elasticmachine]

Pinging @elastic/es-core-infra

[original-brownbear]

Is this a reasonable default?

[original-brownbear]

Adding a serialized boolean to the end of this list feels really dirty, but I couldn't get the BwC tests to pass when adding a new byte to the message (even when wrapping writing that boolean in a if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {). Maybe I'm making a mistake here?

if (in.getVersion().onOrAfter(Version.V_7_0_0_alpha1)) {`
 autoCreateIndex = in.readBoolean();
}

did not work for me in BwC at least.

[original-brownbear]

Maybe this can be resolved in a different way since the issues mentions users will have to update their templates when using the new auto create mode and BwC isn't really an issue?

[original-brownbear]

@jasontedor can you take a quick look here whenever you have a sec?

I'm mainly looking for confirmation that this is going in the right direction functionally and for answers for my two questions (the default + the serialization BwC).

If this is what we want, then I think only integration tests + fixing the watcher compatibility are left (Watcher validates that the auto create pattern allows for its indexes to be created, which fails if the setting is just _template, so that validation will need some additional logic).

[original-brownbear]

@jasontedor ping (if you have a sec :))

[nik9000]

I think this should wait until we have worked out #21105.