Trace a REST call with Kamon with OpenTracing support

TraceRestCall.scala

private def uuid = java.util.UUID.randomUUID.toString

def trace = Directive[Unit] {
  innerRoute ⇒ ctx ⇒ {
    val traceId = s"req-$uuid"
    val textMap = readOnlyTextMapFromHeaders(ctx.request.headers)
    val incomingSpanContext = Kamon.extract(Format.HttpHeaders, textMap)
    val serverSpan = addDefaultTags(Kamon.buildSpan(ctx.request.uri.path.toString))
      .asChildOf(incomingSpanContext)
      .withSpanTag("myTraceId", traceId)
      .startActive()

    val innerRouteResult = innerRoute(ctx)(ctx)
    innerRouteResult.onComplete(_ => serverSpan.finish())(CallingThreadExecutionContext)
    serverSpan.deactivate()

    innerRouteResult.map {
      case Complete(res) =>
        Complete(res.addHeader(RawHeader("x-trace-token", serverSpan.context().traceID.string)))
      case o => o
    }(CallingThreadExecutionContext)
  }
}

def readOnlyTextMapFromHeaders(headers: Seq[HttpHeader]): TextMap = new TextMap {
  private val headersMap = headers.map { h => h.name -> h.value }.toMap
  override def get(key: String): Option[String] = headersMap.get(key)

  override def put(key: String, value: String): Unit = ???
  override def values: Iterator[(String, String)] = headersMap.iterator
}