fix(prometheus_remote_write source): ignore conflicting metadata instead of returning HTTP 400

[elohmeier]

Summary

Fix HTTP 400 errors when Prometheus sends conflicting metric metadata by aligning Vector behavior with Prometheus/Thanos (silently ignore conflicts, continue processing timeseries data).

Vector configuration

[sources.prometheus_remote_write]
type = "prometheus_remote_write"
address = "0.0.0.0:8001"

How did you test this PR?

I've collected HTTP payloads from a Prometheus server configured to remote write to the Vector source via mitmproxy and replayed them to test.

Prometheus sent metadata payloads with e.g. this (protobuf/snappy encoded) v1 remote write Payload:

metadata:
- type: GAUGE
  metricFamilyName: go_memstats_alloc_bytes
  help: Number of bytes allocated and still in use.
- type: COUNTER
  metricFamilyName: go_memstats_alloc_bytes
  help: Total number of bytes allocated, even if freed.
- type: GAUGE
  metricFamilyName: go_memstats_alloc_bytes
  help: Number of bytes allocated in heap and currently in use. Equals to /memory/classes/heap/objects:bytes.

Which would lead to this error in Vector without our fix:

2025-09-13T08:33:22.096665Z  WARN source{component_kind="source" component_id=remote_write component_type=prometheus_remote_write}:http-request{method=POST path=/}: vector::internal_events::http: Received bad request. error=Could not decode write request: multiple metric kinds given for metric name `go_memstats_alloc_bytes` error_code=http_response_400 error_type="request_failed" error_stage="receiving" http_code=400 internal_log_rate_limit=true
{"error":"Could not decode write request: multiple metric kinds given for metric name `go_memstats_alloc_bytes`","error_code":"http_response_400","error_stage":"receiving","error_type":"request_failed","host":"mac","http_code":"400","internal_log_rate_limit":true,"message":"Received bad request.","metadata":{"kind":"event","level":"WARN","module_path":"vector::internal_events::http","target":"vector::internal_events::http"},"pid":87279,"source_type":"internal_logs","timestamp":"2025-09-13T08:33:22.096638Z","vector":{"component_id":"remote_write","component_kind":"source","component_type":"prometheus_remote_write"}}

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

• Somewhat related: Prometheus Remote‑Write Metadata Handling #22825

Notes

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[elohmeier]

Added missing authors to changelog file.

[elohmeier]

Rebased onto latest master and resolved the conflicts.

[elohmeier]

@pront - is there anything I can do to get this over the finish line? This is currently affecting a larger client unfortunately.