[Core] Logs in JSON layout follow ECS format

[mshustov]

Log records emitted in JSON layout contain meta key with additional information (HTTP request/response, process identifier) attached:

kibana/src/core/server/logging/layouts/json_layout.test.ts

Lines 99 to 104 in ab92bbb

	meta: {
	version: {
	from: 'v7',
	to: 'v8',
	},
	},

This structure doesn't follow ECS format. The logging system should restructure additional information from meta key to ECS format as close as possible:
before:

meta: {
  http: {
    request: {
      method: 'GET'
    },
  }
}

after:

http: {
  request: {
    method: 'GET'
  },
}

[elasticmachine]

Pinging @elastic/kibana-core (Team:Core)

[lukeelmers]

The logging system should restructure additional information from meta key to ECS format as close as possible:

Looking at this more closely, it seems we are already merging the meta with the rest of the json log, however we should still be trying to nudge devs toward including proper ECS-compliant meta in their logs.

We discussed the following plan to address this:

• We will make a full set of ECS typings to enforce on the meta field, with a catch-all to accommodate custom fields
  • Could consider forcing everyone to include the ECS version in their meta, will look into how many problems this would cause
  • Long-term we will consider whether it's worth officially supporting ECS types out of core
• Look into renaming meta to something more obvious like ecsMeta & audit usage of log meta across Kibana
• Include special handling for "reserved" keys that should not be overwritten, like timestamp, message, log level
  • In some cases, like someone putting an error object in the meta, preserving the message could be important
  • We should look at whether we can somehow "quarantine" conflicting items in their own custom key so they don't get lost