feat(prometheus_scrape source): Add HTTP redirect following support

[XYenon]

Summary

Add HTTP redirect following support to prometheus_scrape and http_client sources with configurable follow_redirects and max_redirects options.

• prometheus_scrape: follow_redirects defaults to true (matches Prometheus behavior)
• http_client: follow_redirects defaults to false (preserves original behavior)

Redirect behavior follows HTTP spec:

• 301/302/303 redirects change non-HEAD requests to GET
• 307/308 redirects preserve method and body
• Timeout applies to entire redirect chain
• Metrics use original request URL to avoid cardinality explosion

Vector configuration

[sources.prometheus]
type = "prometheus_scrape"
endpoints = ["http://localhost:9090/metrics"]
follow_redirects = true   # default
max_redirects = 5         # default

[sources.http]
type = "http_client"
endpoint = "http://localhost:8080/api"
follow_redirects = false  # default
max_redirects = 5         # default

How did you test this PR?

• Added test_prometheus_follows_redirects unit test that verifies redirect following works and metrics use original URL
• Updated all existing tests with new config fields

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: prometheus_scrape cannot follow HTTP redirects #24416

Notes

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[github-actions]

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[XYenon]

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