feat(codecs): Add inject_metadata to VRL decoder + exec source support

[thomasqueirozb]

Summary

Adds an inject_metadata option to the vrl codec. When enabled, sources can call Decoder::with_metadata_template to pre-populate the synthetic event with per-request context before the VRL program executes, making that context readable via %-prefixed paths during decoding.

The exec source is the first consumer: it injects %exec.host and %exec.command at build time, so VRL programs can read them with string!(%exec.host) etc.

Vector configuration

Basic — read injected metadata inside VRL decoder:

sources:
  exec_source:
    type: exec
    mode: scheduled
    scheduled:
      exec_interval_secs: 5
    command:
      - echo
      - '{"level":"info","msg":"hello from exec"}'
    framing:
      method: newline_delimited
    decoding:
      codec: vrl
      vrl:
        inject_metadata: true
        source: |
          . = parse_json!(string!(.message))
          .decoder_saw_host = get!(%exec.host, [])
          .decoder_saw_command = get!(%exec.command, [])
sinks:
  console:
    type: console
    inputs: [exec_source]
    encoding:
      codec: json

Collision test — VRL can read and overwrite the injected value within the program:

sources:
  exec_source:
    type: exec
    mode: scheduled
    scheduled:
      exec_interval_secs: 5
    command: [echo, "ignored payload"]
    framing:
      method: newline_delimited
    decoding:
      codec: vrl
      vrl:
        inject_metadata: true
        source: |
          .injected_host_before_override = string!(%exec.host)
          %exec.host = "overridden-by-vrl"
          .injected_host_after_override = string!(%exec.host)
          . = {
            "injected_host_before_override": .injected_host_before_override,
            "injected_host_after_override": .injected_host_after_override,
          }

transforms:
  inject_meta:
    inputs: [exec_source]
    type: remap
    source: |
      .meta = %

sinks:
  console:
    type: console
    inputs: [inject_meta]
    encoding:
      codec: json

Output confirms injected_host_after_override: "overridden-by-vrl" and that meta.host == "overridden-by-vrl" too.

How did you test this PR?

• Unit tests: cargo nextest run -E 'test(/vrl/)' (8 tests pass including two new with_metadata_template tests)
• Live: ran both configs above against the built binary

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

• Related: feat(splunk_hec source): support second-stage framing and decoding #25312 (splunk_hec second-stage decoder — planned consumer of this API)
• Related: Deserializer::parse trait is not signal-agnostic due to log_namespace parameter #25044 (Deserializer::parse trait refactor)

[thomasqueirozb]

If there is demand to support metadata on the VRL decoder for more sources this PR can be revived