feat(prometheus_remote_write source): add path configuration option

[elohmeier]

Summary

• allow the prometheus_remote_write source to accept requests on a configurable URL path
• default to / to preserve existing behaviour
• add tests ensuring both custom-path success and wrong-path rejection

Vector configuration

[sources.prom_remote_write]
type = "prometheus_remote_write"
address = "0.0.0.0:9090"
path = "/api/v1/write"

How did you test this PR?

cargo test -p vector --lib sources::prometheus::remote_write::test

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

Please merge #23773 first. I'll then rebase this PR and resolve the conflict.

Notes

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[pront]

This will now pass / by default, which is different than before. Is the behavior ultimately the same?