feat(vlinsert): add vlinsert stream field limit

[bearslyricattack]

No description provided.

[Copilot]

Pull request overview

Adds an optional per-stream-label value row limiter to the Loki JSON ingest path in vlinsert, preventing more than N log rows per configured label value within a time window.

Changes:

• Introduces app/vlinsert/streamfieldlimit package implementing a sharded in-memory windowed row limiter driven by CLI flags.
• Applies the limiter during Loki JSON request parsing to drop rows beyond the per-value quota.
• Adds unit tests for the limiter itself and an integration-style test for Loki JSON parsing with the limiter enabled.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
app/vlinsert/streamfieldlimit/limiter.go	New sharded, windowed row limiter with flag-based configuration.
app/vlinsert/streamfieldlimit/limiter_test.go	Unit tests for limiter behavior and window reset.
app/vlinsert/loki/loki_json.go	Integrates limiter into Loki JSON parsing to cap ingested rows per configured label value.
app/vlinsert/loki/loki_json_test.go	Adds coverage validating row dropping behavior when the limiter is enabled.