feat(reduce transform): Add max_bytes limit to flush groups by size

[saswatamcode]

Summary

This commit,

• Adds a max_bytes option to reduce transform. When set, it checks if an incoming event would bring the group's accumulated size over the limit, and if it does, it flushes the current group and starts a new one with the event. If a single event exceeds the limit on its own, it's still emitted as a group of one.
• Adds a byte_limit_flushed_total counter metric that increments each time a flush is triggered by the byte limit

Quite new to Rust here, so would appreciate any feedback!

Vector configuration

  [transforms.reduce_multiline]
  type = "reduce"
  inputs = ["kubernetes_logs"]
  group_by = [
    "kubernetes.pod_name",
    "kubernetes.container_name",
  ]
  max_bytes = 262144
  expire_after_ms = 5000

  [transforms.reduce_multiline.merge_strategies]
  message = "concat_newline"
  "kubernetes.pod_name" = "retain"
  "kubernetes.container_name" = "retain"
  "kubernetes.namespace" = "retain"
  "kubernetes.pod_labels" = "retain"

How did you test this PR?

Unit tests

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

N/A

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[saswatamcode]

I have read the CLA Document and I hereby sign the CLA

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