Migrate elasticsearch client to the new undici http client

[mshustov]

Blocked on elastic/elasticsearch-js#1740

#108387 follow-up

elaticserarch-js client starting from v8.0 uses undici as a default HTTP client. undici has been rewreitten from scratches to supersede the standard http module of nodejs. Undici has significantly lower memory pressure and runtime overhead, comparing to the standard http module.

During the client version upgrade, I decided not to use undici client in the ES client for the following reasons:

• It is not very safe to make such an impactful change of a low-level dependency just a few days before the v8.0 FF date. It's better to test if during the development process thoroughly.
• APM agent is going to add support for undici client after v8.0 support v8.x of @elastic/elasticsearch apm-agent-nodejs#2355 (comment)
• undici client doesn't support keepAlive and maxSockets options https://github.com/elastic/elastic-transport-js/blob/main/src/connection/UndiciConnection.ts#L224-L226 We need to investigate what undici suggest using instead and how it can affect Core elasticsearch service API.
• I saw a few errors in the Kibana logs (like received 'get' method, expects on of ['GET', 'POST'...)

[elasticmachine]

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

[gsoldevila]

There's an initiative to monitor and track the number of open sockets with the standard 'http' client. The goal is to have some visibility on that value, and to be able to fine-tune the value of maxSockets (which is set to Infinity at the time of writing).

If/when we resume efforts to test (and to switch to) the undici implementation, it is worth noting that the undici.Pool used by elasticsearch-js's UndiciConnection has a get stats() method, that should allow to continue tracking the number of open sockets.

Then, in terms of configuration:

undici client doesn't support keepAlive and maxSockets options

I believe undici is keepAlive by default, and it does support agent.connections: number which allows defining an upper limit to the number of open connections in the pool (or so it seems).

[trentm]

• APM agent is going to add support for undici client after v8.0

Undici support (and testing using the UndiciConnection with @elastic/elasticsearch) is now in the APM agent: [email protected] (elastic/apm-agent-nodejs#2872)