Ability to filter and properly format logs received from Slf4J API

[nartamonov]

I use zio-logging with slf4j-bridge so that any logs from legacy non-ZIO libraries which use Slf4j API go through zio-logging stuff. So I expect the functionality of zio-logging can be applied to logs received from legacy libraries.

Currently slf4j-bridge stores original names of Slf4J loggers into log spans:

// Excerpts from org.slf4j.impl.ZioLogger

private def run(f: ZIO[Any, Nothing, Unit]): Unit =
  factory.run {
    ZIO.logSpan(name)(f)
  }

// ...

// Any call to Slf4j API goes throught run()
override def info(msg: String): Unit =
  run(ZIO.logInfo(msg))

This leads to significant limitations:

• We can't filter logs received from Slf4J API via standard LogFilter.logLevelByName because it depends on the LoggerNameExtractor.trace that currently doesn't extract the original names of Slf4J loggers. In all cases the name of the logger will be org.slf4j.impl.ZioLogger. That's useless.
• We can't make a LogFormat that extracts the name of the Slf4J logger for the same reason: LoggerNameExtractor.trace can't do it.

Moreover, users of zio-logging can't write their own version of LoggerNameExtractor that can extract Slf4J logger names because LoggerNameExtractor does not have access to attached log spans:

trait LoggerNameExtractor {
  def apply(
    trace: Trace,
    context: FiberRefs,
    annotations: Map[String, String]
  ): Option[String]
}

Proposals

I see two possible solutions:

1. We can adjust interface LoggerNameExtractor to give it access to log spans.
2. We can store original name of the Slf4j logger in annotation instead of span, something like:

  private def run(f: ZIO[Any, Nothing, Unit]): Unit =
    factory.run {
      ZIO.logAnnotation("slf4j-logger-name", name)(f)
    }

[justcoon]

@nartamonov, thank you for report and proposal,

yes, I think you are right, considering current design of zio-logging, LogAnnotation is probably better way for log name propagation

i created PR #599