chore(performance,vrl): Add RFC for "LLVM Backend for VRL"

[pablosichert]

Closes #10517.

Rendered version 👀.

Some preliminary results from my local machine (executing the script 1M times each):

.hostname = "vector"

if .status == "warning" {
    .thing = upcase(.hostname)
} else if .status == "notice" {
    .thung = downcase(.hostname)
} else {
    .nong = upcase(.hostname)
}

.matches = { "name": .message, "num": "2" }

Traverse: 1.653175s
VM: 715.790583ms
LLVM: 608.121458ms

---

.foo = string("hi!")

Traverse: 248.525166ms
VM: 102.147416ms
LLVM: 41.894708ms

---

.key = "foo"

Traverse: 275.768333ms
VM: 57.462375ms
LLVM: 33.156291ms

---

.num = 123

Traverse: 270.735291ms
VM: 43.291541ms
LLVM: 27.7585ms

---

if .status == 123 {
    .foo = "bar"
}

Traverse: 241.31875ms
VM: 50.283333ms
LLVM: 31.510625ms

---

Additional material: slides for the presentation on LLVM Backend for VRL.

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[StephenWakely]

I think it could be worth adding a section to discuss calling functions in the VRL stdlib. We need to give some thought to how it will handle optional and named parameters. I'm not sure what the best solution would be here.

[pablosichert]

@StephenWakely I agree that we still need to figure out the specifics of handling function parameters. However, I don't think the implementation details matter too much to the general approach lined out here other than "we will find a solution". If there's demand to discuss the details of these, we can add another RFC.

I added it to the action items: 1e8b6ab.

[JeanMertz]

This is a great RFC, nice work @pablosichert.

I did leave some remarks and questions inline. Additionally, I'd like there to be a section specifically around testing (how we will test, what does this require us to have in terms of testing infrastructure, what parts of this are difficult to test and require manual checks during PR reviews).

(I know there are two items on testing in the "Plan Of Attack" section, but I'd like that to be expanded into its own section in the actual RFC, with an explanation on why we need certain tests, etc)

[StephenWakely]

This is a great start to a very promising project!

[JeanMertz]

🎉