fix: Investigate recent api response changes

[slim-bean]

What this PR does / why we need it:
This PR fixes an issue where /loki/api/v1/query_range responses were incorrectly returning Content-Type: text/plain; charset=utf-8 instead of application/json; charset=UTF-8. This caused client-side errors due to unexpected MIME types. The fix explicitly sets the Content-Type header for JSON responses.

Which issue(s) this PR fixes:
Fixes #

Special notes for your reviewer:
The bug was introduced in December 2024 with the addition of Parquet support, where the Content-Type for JSON responses was inadvertently omitted. It became visible in the last 4 weeks due to recent changes wiring up the v2 engine handler into the query frontend, which increased traffic through the affected code path. The fix is a single line to set the header, consistent with other JSON response handling in the codebase.

Checklist

• Reviewed the CONTRIBUTING.md guide (required)
• Documentation added
• Tests updated
• Title matches the required conventional commits format, see here
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Slack Thread

 

[cursor]

Cursor Agent can help with this pull request. Just @cursor in comments and I'll start working on changes in this branch.
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[slim-bean]

@cursor can you check if any of the other query endpoints also need to be fixed?

[cursor]

Great news! I've checked all query endpoints and the fix in serialize.go covers all query endpoints that were affected. Here's what I found:

✅ Fixed by this PR

All query endpoints flow through NewQuerierHTTPHandler() which wraps handlers with NewSerializeHTTPHandler() (the handler I just fixed):

• /loki/api/v1/query_range - Range queries
• /loki/api/v1/query - Instant queries
• /loki/api/v1/labels - Label queries
• /loki/api/v1/label/{name}/values - Label values
• /loki/api/v1/series - Series queries
• /loki/api/v1/index/stats - Index stats
• /loki/api/v1/index/shards - Shards
• /loki/api/v1/index/volume - Volume
• /loki/api/v1/detected_fields - Detected fields
• /loki/api/v1/patterns - Patterns
• /loki/api/v1/detected_labels - Detected labels

Reference: pkg/querier/handler.go lines 139-141

✅ No Issues Found

Query Frontend Transport Handler (pkg/lokifrontend/frontend/transport/handler.go):

• Uses roundTripper.RoundTrip() which returns complete HTTP responses
• Copies headers from the roundtripper's response
• Content-Type header comes from codec's EncodeResponse() which sets it correctly

Internal encoding functions:

• prometheus.go - The encodeTo() method is only called by encode() which sets Content-Type: application/json
• All other internal encoding paths use the codec's EncodeResponse() which properly sets headers

Conclusion

The single-line fix in serializeHTTPHandler.ServeHTTP() resolves the issue for all affected query endpoints. No additional changes needed! 🎉

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