proxy: normalize wrapped Loki responses when resultType is missing

[szibis]

Summary

This fixes intermittent Grafana Loki datasource failures reporting no resultType found in Logs Drilldown.

Root cause was wrapAsLokiResponse passing backend payloads through even when data.resultType was missing (or when payloads used top-level results). Grafana's Loki plugin then failed to decode those responses.

Changes

• normalize wrapped payloads to always provide Loki data.resultType (fallback from call context)
• ensure Loki data.result exists (fallback empty array)
• map top-level results into Loki result while keeping legacy results for compatibility
• add regression tests for both payload shapes

Validation

• go test ./internal/proxy -count=1
• manual Grafana reproduction showed failing datasource query responses with error: no resultType found from plugin

[github-actions]

PR Quality Report

Compared against base branch main.

Coverage and tests

Signal	Base	PR	Delta
Test count	1807	1809	2
Coverage	89.3%	89.3%	0.0% (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	1277.0 ns/op	1286.0 ns/op	+0.7% (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	1621.0 ns/op	1628.0 ns/op	+0.4% (stable)
QueryRange cache-bypass memory	271.0 B/op	270.0 B/op	-0.4% (stable)
QueryRange cache-bypass allocations	7.0 allocs/op	7.0 allocs/op	0.0% (stable)
Labels cache-hit CPU cost	681.5 ns/op	682.4 ns/op	+0.1% (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	817.7 ns/op	840.1 ns/op	+2.7% (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	131468.0 req/s	113525.0 req/s	-13.6% (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.