[7.17](backport #42966) fix(libbeat): mitigate race condition in ratelimit processor

[mergify]

Proposed commit message

This PR fixes race conditions on the ratelimit processor. The race conditions can only manifest with configurations that have a global ratelimit processor and multiple inputs. Fixed by adding a mutex to the bucket struct.

During benchmarking, I also noticed an unnecessary allocation occurring in the hot path. Removing this allocation resulted in a 25% performance improvement. While locks have an inherent cost, in this scenario, the performance gain offsets that cost.

Checklist

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

How to test this PR locally

Run filebeat with a global ratelimit processor under the race detector:

$ cd x-pack/filebeat; go build -race .
$ GORACE="halt_on_error=1" ./filebeat -c ratelimit.yml -d '*' -v -e

filebeat:
  inputs:
    - type: benchmark
      enabled: true
      message: "bench1"
      eps: 100
    - type: benchmark
      enabled: true
      message: "bench2"
      eps: 100
    - type: benchmark
      enabled: true
      message: "bench3"
      eps: 100
    - type: benchmark
      enabled: true
      message: "bench4"
      eps: 100
processors:
- rate_limit:
   limit: "1/s"
output.console:
  pretty: true
logging:
  level: debug
  selectors:
    - "*"
path.home: /tmp/filebeat

---

This is an automatic backport of pull request #42966 done by [Mergify](https://mergify.com).

[mergify]

Cherry-pick of acce29c has failed:

On branch mergify/bp/7.17/pr-42966
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Changes to be committed:
	modified:   CHANGELOG.next.asciidoc
	modified:   libbeat/processors/ratelimit/rate_limit_test.go

Unmerged paths:
  (use "git add <file>..." to mark resolution)
	both modified:   libbeat/processors/ratelimit/token_bucket.go

To fix up this pull request, you can check it out locally. See documentation: https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/reviewing-changes-in-pull-requests/checking-out-pull-requests-locally

[botelastic]

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[faec]

broken changelog merge should be fixed (along with the explicit broken merges below)