app/vlinsert/loki: handle empty structured metadata

[jwmay2012]

Grafana Alloy can send structured metadata whose extracted value is empty. Loki's LabelPairAdapter uses implicit-presence proto3 string fields, so generated encoders do not serialize name or value when the corresponding string is empty. VictoriaLogs currently treats that normal proto3 omission as malformed and rejects the complete push with missing name or missing value.

The resulting error is:

cannot decode PushRequest: cannot unmarshal Stream: cannot unmarshal Entry: cannot unmarshal StructuredMetadata: missing value

This appears to be an unintended side effect of the streaming protobuf parser optimization, released in v1.40.0. A sibling OpenTelemetry decoder introduced by the same optimization pattern was subsequently changed to skip attributes with omitted keys rather than reject the request. The Loki decoder already intentionally ignores a metadata pair after decoding when either its name or value is empty.

Treat omitted fields as their empty-string defaults and continue ignoring metadata pairs unless both values are non-empty. Malformed encoded fields still return errors.

Adds an end-to-end protobuf regression test covering an omitted name, an omitted value, an empty pair, and a valid pair in the same request, plus a changelog entry.

Tested with:

• go test ./app/vlinsert/loki -count=1
• make check-all
• make test-full
• make apptest

[cubic-dev-ai]

1 issue found across 3 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="docs/victorialogs/CHANGELOG.md">

<violation number="1" location="docs/victorialogs/CHANGELOG.md:38">
P3: This new BUGFIX entry has no references link, unlike every other FEATURE/BUGFIX line in the changelog, which ends with a `See [#NNNN](...)` link (e.g. lines 39-41, or line 293 for the prior Loki ingestion fix). Add a references link using the current PR as the reference.</violation>
</file>

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[cubic-dev-ai]

All reported issues were addressed across 3 files (changes from recent commits).

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