Correlation id support

See the blog for introduction.

Currently supports monix & logback.

Generic usage:

• add the dependency: "com.softwaremill.correlator" %% "monix-logback-http4s" % "0.1.8" to your project
• create an object extending the CorrelationIdDecorator class, e.g. object MyCorrelationId extends CorrelationIdDecorator()
• call MyCorrelationId.init() immediately after your program starts (in the main() method)
• create an implicit instance of CorrelationIdSource for given T from which you want to extract correlation id
• wrap any T => Task[R] function with MyCorrelationId.withCorrelationId, so that a correlation id is extracted from the argument (using the defined CorrelationIdSource), or a new one is created.
• you can access the current correlation id (if any is set) using MyCorrelationId.apply() or MyCorrelationId.applySync().

For http4s integration:

• add the dependency: "com.softwaremill.correlator" %% "monix-logback-http4s" % "0.1.8" to your project
• create an object extending the CorrelationIdDecorator class, e.g. object MyCorrelationId extends CorrelationIdDecorator()
• call MyCorrelationId.init() immediately after your program starts (in the main() method)
• wrap your HttpRoutes[Task] with Http4sCorrelationMiddleware(MyCorrelationId).withCorrelationId, so that a correlation id is extracted from the request (using the provided header name), or a new one is created.
• you can access the current correlation id (if any is set) using MyCorrelationId.apply() or MyCorrelationId.applySync().

Logging each request with corresponding correlationId can be done in following way:

def loggingMiddleware[T, R](
                           service: HttpRoutes[Task],
                           logStartRequest: Request[Task] => Task[Unit] = req =>
                             Task(MyLogger.debug(s"Starting request to: ${req.uri.path}"))
                         ): HttpRoutes[Task] = Kleisli{ req: Request[Task] =>
    val setupAndService = for {
      _ <- logStartRequest(req)
      r <- service(req).value
    } yield r
    OptionT(setupAndService)
}

val middleware = Http4sCorrelationMiddleware(correlationIdDecorator)
middleware.withCorrelationId(loggingMiddleware(service))