fix(sources): collect headers for logs in opentelemetry source with use_otlp_decoding set to true

[ozanichkovsky]

Summary

This PR expands the existing header-enrichment logic in the opentelemetry source so that headers are applied consistently to logs across both decoding modes (use_otlp_decoding enabled or disabled).

What the PR Does
Ensures headers are also applied when use_otlp_decoding: true, where enrichment was previously skipped entirely.

Verification

The changes were tested using a custom local build derived from this PR, confirming that headers are now correctly attached to logs (both decode modes).

Vector configuration

How did you test this PR?

Change Type

• Bug fix
• New feature
• 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

Closes: #24306

Notes

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[github-actions]

All contributors have signed the CLA ✍️ ✅
_{Posted by the CLA Assistant Lite bot.}

[ozanichkovsky]

recheck

[ozanichkovsky]

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

[ozanichkovsky]

recheck

[pront]

Thanks @ozanichkovsky (FYI I just happened to see this, I am on a leave so ping the Vector team instead to drive this forward)

[ozanichkovsky]

Thanks @pront. I don't know for sure if there is a way to properly tag Vector team, so I'm tagging @thomasqueirozb and
@michaelcretzman I found as reviewers in other closed PRs.

[ozanichkovsky]

Maybe I can also tag @jszwedko

[thomasqueirozb]

Hey @ozanichkovsky, pinging @vectordotdev/vector should work (without the backticks)

[thomasqueirozb]

These changes make sense. Thanks for your contribution!

[ozanichkovsky]

@thomasqueirozb I updated the changelog to actually say that it is for logs only. I tried to implement this for all event types first, but it would rather be a new feature implementation, not a bug fix.

[ozanichkovsky]

@vectordotdev/vector should be ready to pass all the checks.

[ozanichkovsky]

@thomasqueirozb I unfortunately can't run all checks, so merging is still blocked until something is done by the vector team.