add support for structured metadata

[plandem]

Update log_builder.ts
Added support for for structured metadata https://grafana.com/docs/loki/latest/reference/loki-http-api/#ingest-logs

Now its possible to pass structured metadata as simple as adding object meta, e.g.:

logger.info({ foo: 'bar', meta: { id: "123456" }}, "hello world");

[Julien-R44]

Hey, thanks for the PR

I think it's going to be a breaking change? Maybe some people use a meta key today without specifically wanting it to be a structured metadata?

Maybe this feature should be opt-in? For example enableStructuredMetadata in the options? or a structuredMetadataKey to specify the key that would be used for structured metadata? What do you think?

[plandem]

not breaking changes in general, but yeah, may impact in same cases. Probably you are right and its better to make an option of transport, e.g. like:

const transport = pino.transport<LokiOptions>({
  target: "pino-loki",
  options: {
    batching: true,
    interval: 5,
    metadataKey: 'meta',
    host: 'https://my-loki-instance:3100',
    basicAuth: {
      username: "username",
      password: "password",
    },
  },
});

and later to use that option.

Can you do it? I decided to switch for pull in model and use promtail, so not sure I will use/need it now.