Add JSON body expansion to log attributes with configurable threshold

[RaphaelManke]

Summary

This PR adds automatic expansion of JSON-formatted log bodies into individual log attributes, with a configurable threshold to prevent attribute explosion. When a log message body is valid JSON object, its key-value pairs are extracted and added as separate attributes to the OTLP log record.

Key Changes

• New helper functions in src/otlp/log_mutations.rs:

  • json_value_to_any_value(): Converts serde_json values to OpenTelemetry AnyValue types, handling strings, booleans, integers, floats, nulls, and complex types (objects/arrays as stringified JSON)
  • try_expand_json_body_to_attributes(): Attempts to parse log body as JSON object and convert to KeyValue attributes, respecting a configurable maximum attribute count

• Integration in map_logs_to_otlp():

  • JSON expansion is applied only to non-platform logs (user function logs)
  • Respects a configurable threshold via DASH0_LOG_JSON_BODY_MAX_ATTRIBUTES environment variable
  • Original log body is preserved unchanged

• New configuration in src/config/user.rs:

  • log_json_body_max_attributes(): Reads DASH0_LOG_JSON_BODY_MAX_ATTRIBUTES environment variable with a default of 25
  • Returns 0 to disable expansion entirely

• Comprehensive test coverage:

  • Unit tests for json_value_to_any_value() covering all JSON types
  • Unit tests for try_expand_json_body_to_attributes() covering edge cases (empty objects, threshold boundaries, invalid JSON)
  • Integration tests for map_logs_to_otlp() covering various scenarios: JSON expansion, non-JSON bodies, platform logs, Node.js prefix handling, and threshold enforcement
  • Configuration tests for the new environment variable

Implementation Details

• JSON expansion only occurs for function logs (not platform logs like platform.start)
• Objects exceeding the configured attribute limit are not expanded (body remains as-is)
• Complex JSON types (nested objects, arrays) are stringified as attribute values
• Null values are converted to the string "null"
• The feature is disabled by default when DASH0_LOG_JSON_BODY_MAX_ATTRIBUTES=0

https://claude.ai/code/session_01QyWWPfMHcJ4S4wWJNUAwj6