Performance counter layout renderer parse bug

Nlog version: 4.6.7

Platform: .Net 4.7

Current NLog config (xml or C#, if relevant)

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" internalLogFile="c:/logs/nlog.log" internalLogLevel="Trace">
    <variable name="facilityName" value="BppprCalc" />
    <targets async="true">
        <target name="graylogCounters" xsi:type="GraylogHttp" facility="${facilityName}" graylogServer="http://graylog" graylogPort="12201">
        <parameter name="processor_total" layout="${performancecounter:Category=Processor:Counter=% Processor Time:Instance=_Total}" />
        <parameter name="requests_per_second" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Requests/Sec:Instance=${gdc:InstanceName}}" />
        <parameter name="requests_in_queue" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Requests in Application Queue:Instance=${gdc:InstanceName}}" />
        <parameter name="errors_total_per_second" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Errors Total/Sec:Instance=${gdc:InstanceName}}" />
      </target>
    </targets>
    <rules>
      <logger name="counters" level="debug" writeTo="graylogCounters" final="true"/>
    </rules>
  </nlog>

• What is the current result? Layout performance counter gives an error while parsing(I think) gdc:. I have putted variables in gdc and value is all right. This is internal logs of setup.

2019-08-30 13:19:16.5796 Warn Exception in layout renderer. Exception: System.InvalidOperationException: Экземпляр '${gdc' не существует в указанной категории.
  в System.Diagnostics.CounterDefinitionSample.GetInstanceValue(String instanceName)
  в System.Diagnostics.PerformanceCounter.NextSample()
  в NLog.LayoutRenderers.PerformanceCounterLayoutRenderer.GetValue()
  в NLog.LayoutRenderers.PerformanceCounterLayoutRenderer.Append(StringBuilder builder, LogEventInfo logEvent)
  в NLog.LayoutRenderers.LayoutRenderer.RenderAppendBuilder(LogEventInfo logEvent, StringBuilder builder)

In english the result is the gdc do not exist in selected category.

• What is the expected result? performance counters layout renderer works well and can get data from gdc.
• I also tried to add a variable into target and reconfigure my config at application startup(global.asax).

<nlog xmlns="http://www.nlog-project.org/schemas/NLog.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" internalLogFile="c:/logs/nlog.log" internalLogLevel="Trace">
    <variable name="facilityName" value="BppprCalc" />
    <variable name="instanceName" value="DefaultInstance" />
    <targets async="true">
      <target name="graylogCounters" xsi:type="GraylogHttp" facility="${facilityName}" graylogServer="http://graylog" graylogPort="12201">
        <parameter name="processor_total" layout="${performancecounter:Category=Processor:Counter=% Processor Time:Instance=_Total}" />
        <parameter name="requests_per_second" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Requests/Sec:Instance=${instanceName}}" />
        <parameter name="requests_in_queue" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Requests in Application Queue:Instance=${instanceName}}" />
        <parameter name="errors_total_per_second" layout="${performancecounter:Category=ASP.NET Apps v4.0.30319:Counter=Errors Total/Sec:Instance=${instanceName}}" />
      </target>
    </targets>
    <rules>
      <logger name="counters" level="debug" writeTo="graylogCounters" final="true"/>
    </rules>
  </nlog>

code at startup

NLog.LogManager.Configuration.Variables["instanceName"] = "_LM_W3SVC_1_ROOT_BppprCalc";
NLog.LogManager.ReconfigExistingLoggers();

and there is another mistake

 2019-08-30 14:04:12.7192 Warn Exception in layout renderer. Exception: System.InvalidOperationException: Экземпляр 'DefaultInstance' не существует в указанной категории.
    в System.Diagnostics.CounterDefinitionSample.GetInstanceValue(String instanceName)
    в System.Diagnostics.PerformanceCounter.NextSample()
    в NLog.LayoutRenderers.PerformanceCounterLayoutRenderer.GetValue()
    в NLog.LayoutRenderers.PerformanceCounterLayoutRenderer.Append(StringBuilder builder, LogEventInfo logEvent)
    в NLog.LayoutRenderers.LayoutRenderer.RenderAppendBuilder(LogEventInfo logEvent, StringBuilder builder)

[snakefoot]

@Kirya522 NLog ver. 4.6.8 has been released that allows NLog Layout-logic to work for the Instance-property:

https://www.nuget.org/packages/NLog

[snakefoot]

Created #3579 with example of how to implement NLog Layout magic.