fix: add special logging fields section in the doc

[cindy-peng]

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Fixes #690 🦕
This is to add a small section to explain the special fields in structured logging.

[generated-files-bot]

Warning: This pull request is touching the following templated files:

• README.md - README.md is managed by synthtool. However, a partials file can be used to update the README, e.g.: https://github.com/googleapis/nodejs-storage/blob/main/.readme-partials.yaml

[daniel-sanche]

I believe this only applies to logs written to stdout, when parsed through the Logging Agent. You should make sure that is captured in the message here

[cindy-peng]

Sorry Daniel I missed this comment earlier.

Initially I thought the same, then I looked into code and it looks like we are honoring these special fields and do a special processing from bunyan entry even without redirectToStdout: true. Here is the where the logEntry is formatted: https://github.com/googleapis/nodejs-logging-bunyan/blob/main/src/index.ts#L253.

An example code snippet I was using Bunyan logger without redirectToStdout:

const loggingBunyan = new LoggingBunyan();


 const logger = bunyan.createLogger({
  name: 'my-service',
  streams: [
    {stream: process.stdout, level: 'info'},
    loggingBunyan.stream('info'),
  ],
});


logger.info({ message: 'boom11',  labels: {first: "label-name", count: "2"} , "logging.googleapis.com/trace_sampled": true }, 'this is the actual message');

You can see the special fields (traceSampled and labels here) are being processed and actually promoted to the log entry level:

Maybe we can just leave the doc as this since special fields handling looks like the default behavior?

[daniel-sanche]

Ok interesting, I was thinking from the backend perspective, but I guess nodejs must be doing its own parsing as well. LGTM then