[Querier] Some JSON appears corrupted by label detection

[kelnage]

Describe the bug
When running a query over JSON data that, when parsed, have detected labels that collide with existing index labels, the log lines will appear to be modified such that the JSON key is replaced with key_extracted. These additional 10 characters overwrite the next 10 characters in the log line, invariably causing the JSON to fail to parse.

The behaviour is not consistent when the query is run repeatedly, so sometimes these keys will not be corrupted and sometimes they will.

To Reproduce
Steps to reproduce the behavior: provided via internal Slack

Expected behavior
The log line to be returned as it was ingested into Loki, and successfully parsed by the JSON parser.

Environment:

• Infrastructure: Kubernetes
• Deployment tool: jsonnet

Screenshots, Promtail config, or terminal output
Provided via internal Slack