fix(opetelemetry source): collect headers for logs, metrics, traces with/without use_otlp_decoding

[ozanichkovsky]

Summary

This PR expands the existing header-enrichment logic in the opentelemetry source so that headers are applied consistently to all event types—logs, metrics, and traces—and across both decoding modes (use_otlp_decoding enabled or disabled).

What the PR Does

• Adds header-enrichment support to metrics and traces, which previously ignored configured headers.
• Ensures headers are also applied when use_otlp_decoding: true, where enrichment was previously skipped entirely.
• Unifies the enrichment behavior so users can reliably attach metadata for routing, filtering, or downstream correlation regardless of event type or decoding method.

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),
• metrics,
• traces.

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]

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[ozanichkovsky]

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[ozanichkovsky]

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[ozanichkovsky]

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