Add GCP complient tracing data to JSON logging

[brunobat]

A new JSON format option was created:

quarkus.log.file.json.log-format=gcp

When this is used, tracing data is flatten and copied to the top level according to these semantics: https://cloud.google.com/logging/docs/structured-logging#structured_logging_special_fields
The logging level s also changed to severity.

This is an output example:

{
  "timestamp": "2025-10-02T18:29:25.342877813+01:00",
  "sequence": 6092,
  "loggerClassName": "org.jboss.logging.Logger",
  "loggerName": "io.quarkus.it.logging.json.GreetingResource",
  "severity": "INFO",
  "message": "Hello to \"Quarkus\"",
  "threadName": "executor-thread-1",
  "threadId": 123,
  "mdc": {
    "spanId": "0dabdb19210281f8",
    "traceId": "7b5b422a3756128859616cd60feb7e80",
    "sampled": "true"
  },
  "ndc": "",
  "hostName": "xxxxxxxxxxxxxxxxxxxx",
  "processName": "xxxxxxxxxxxxxx.sdkman/candidates/java/21.0.8-tem/bin/java",
  "processId": 79852,
  "spanId": "0dabdb19210281f8",
  "trace": "projects/quarkus-integration-test-logging-json/traces/7b5b422a3756128859616cd60feb7e80",
  "traceSampled": "true"
}

Documentation for JSON logging was also improved.
I chose not to exclude the MDC field because it might contain other relevant data.

• Closes: Group logs by request - GCP #50319

[github-actions]

🎊 PR Preview c3944a9 has been successfully built and deployed to https://quarkus-pr-main-50368-preview.surge.sh/version/main/guides/

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[brunobat]

@gsmet, @geoand, Hey folks, I wonder if one of you could give a review here.

[quarkus-bot]

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