enhancement(prometheus scrape): support request headers

[arfa79]

Summary

Adds configurable HTTP request headers to the prometheus_scrape source. Configured headers are passed to Vector’s shared HTTP client, including support for overriding the default Accept header.

Vector configuration

sources:
  prometheus:
    type: prometheus_scrape
    endpoints:
      - http://localhost:9090/metrics
    headers:
      Accept:
        - application/openmetrics-text
      X-My-Header:
        - custom-value

How did you test this PR?

• cargo fmt --all -- --check
• cargo test --no-default-features --features sources-prometheus-scrape sources::prometheus::scrape::test -- --nocapture — 7 passed
• cargo clippy --no-default-features --features sources-prometheus-scrape --lib -- -D warnings

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. A changelog fragment is included.
• No.

References

Closes #19656

Notes

Implemented with assistance from OpenAI Codex (GPT-5.6)

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1 out of 2 committers have signed the CLA.
✅ (pront)[https://github.com/pront]
❌ @arfa79
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[arfa79]

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

[arfa79]

recheck

[pront]

Taking a look at the failing CLA check

[pront]

@arfa79 did you add the email used for these commits to your github account?

See 19cab1e