fix: relax metadata fallback on empty strict scans and harden extract literals

[szibis]

Summary

• relax detected labels/fields discovery when strict candidate returns a successful-but-empty response
• retry native streams/field-name metadata lookups with relaxed candidates when strict lookup is empty
• harden translator so pattern/extract stages without named captures become line filters instead of invalid extract pipes
• update tests to assert relaxed fallback behavior and literal-extract translation behavior

Why

• Drilldown metadata endpoints can intermittently return empty data when a strict candidate has no rows but a relaxed candidate does
• VictoriaLogs rejects extract "Metrics" because the pattern has no named field placeholder

Validation

• go test ./internal/translator ./internal/proxy
• local compose backend repro against seeded data:
  • strict query ... | json | source_message_bytes="999" now returns non-empty detected_labels and detected_fields
  • repeated calls are stable (zero=0 nonzero=20 for both labels and fields)
  • direct backend query with | extract "Metrics" | stats by (level) rate() still reproduces the parser error, while proxy-side literal extract translation no longer surfaces that parser failure

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	1809	1811	2
Coverage	89.3%	89.2%	-0.1% (stable)

Compatibility

Track	Base	PR	Delta
Loki API	100.0%	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	100.0%	17/17 (100.0%)	0.0% (stable)
VictoriaLogs	100.0%	11/11 (100.0%)	0.0% (stable)

Compatibility components

Track	Component	Base	PR	Delta
Loki API	label_values	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Loki API	labels	4/4 (100.0%)	4/4 (100.0%)	0.0% (stable)
Loki API	metrics	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Loki API	otel	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Loki API	query_range	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Loki API	series	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	detected_fields	11/11 (100.0%)	11/11 (100.0%)	0.0% (stable)
Logs Drilldown	label_values	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	level_volume	2/2 (100.0%)	2/2 (100.0%)	0.0% (stable)
Logs Drilldown	patterns	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	service_logs	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
Logs Drilldown	service_selection	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	detected_fields	4/4 (100.0%)	4/4 (100.0%)	0.0% (stable)
VictoriaLogs	field_values	3/3 (100.0%)	3/3 (100.0%)	0.0% (stable)
VictoriaLogs	index_stats	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	stream_translation	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	synthetic_labels	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)
VictoriaLogs	volume_range	1/1 (100.0%)	1/1 (100.0%)	0.0% (stable)

Performance smoke

Lower CPU cost (ns/op) is better. Lower benchmark memory cost (B/op, allocs/op) is better. Higher throughput is better. Lower load-test memory growth is better. Benchmark rows are medians from repeated samples.

Signal	Base	PR	Delta
QueryRange cache-hit CPU cost	1370.0 ns/op	1355.0 ns/op	-1.1% (stable)
QueryRange cache-hit memory	200.0 B/op	200.0 B/op	0.0% (stable)
QueryRange cache-hit allocations	7.0 allocs/op	7.0 allocs/op	0.0% (stable)
QueryRange cache-bypass CPU cost	1665.0 ns/op	1644.0 ns/op	-1.3% (stable)
QueryRange cache-bypass memory	274.0 B/op	271.0 B/op	-1.1% (stable)
QueryRange cache-bypass allocations	7.0 allocs/op	7.0 allocs/op	0.0% (stable)
Labels cache-hit CPU cost	681.1 ns/op	700.8 ns/op	+2.9% (stable)
Labels cache-hit memory	48.0 B/op	48.0 B/op	0.0% (stable)
Labels cache-hit allocations	3.0 allocs/op	3.0 allocs/op	0.0% (stable)
Labels cache-bypass CPU cost	841.1 ns/op	841.4 ns/op	+0.0% (stable)
Labels cache-bypass memory	53.0 B/op	53.0 B/op	0.0% (stable)
Labels cache-bypass allocations	3.0 allocs/op	3.0 allocs/op	0.0% (stable)
High-concurrency throughput	116883.0 req/s	99368.0 req/s	-15.0% (stable)
High-concurrency memory growth	0.4 MB	0.4 MB	0.0% (stable)

State

• Coverage, compatibility, and sampled performance are reported here from the same PR workflow.
• This is a delta report, not a release gate by itself. Required checks still decide merge safety.
• Performance is a smoke comparison, not a full benchmark lab run.
• Delta states use the same noise guards as the quality gate (percent + absolute + low-baseline checks), so report labels match merge-gate behavior.