NLog GlobalDiagnosticContext should be explictly added using ContextProperties

[snakefoot]

This is a refactoring of #152 by @jeff-zohrab

It is a bad idea to include all NLog GDC-items, as they are often used a global variables across all NLog-targets. This means all kind of random junk will suddenly be sent to the ApplicationInsightsTarget.

Instead this PR allows one to add the wanted global items explicitly using the new ContextProperties:

<nlog>
   <extensions>
      <add assembly="Microsoft.ApplicationInsights.NLogTarget" />
   </extensions>
   <targets>
      <target type="ApplicationInsightsTarget" name="aiTarget" />
           <contextProperty name="appid" layout="${gdc:item=appid}" />
           <contextProperty name="threadid" layout="${threadid}" />
      </target>
   </targets>
   <rules>
      <logger name="*" minlevel="Trace" writeTo="aiTarget" />
   </rules>
</nlog>

One can also add other random stuff like threadid, trace-correlationid, etc.

[snakefoot]

One might consider removing this code:

https://github.com/Microsoft/ApplicationInsights-dotnet-logging/blob/388ff8e884711ffdc6a7ef8cab16a0b136b6c2ac/src/NLogTarget/ApplicationInsightsTarget.cs#L75-L79

As it can be replaced by this config (Property names copied from log4net appender):

      <target type="ApplicationInsightsTarget" name="aiTarget" />
           <contextProperty name="ClassName" layout="${callsite:className=true:fileName=false:methodName=false}" />
           <contextProperty name="FileName" layout="${callsite:className=false:fileName=true:methodName=false}" />
           <contextProperty name="MethodName" layout="${callsite:className=false:fileName=false:methodName=true}" />
           <contextProperty name="LineNumber" layout="${callsite-linenumber}" />
      </target>

And it will actually ensure that NLog will capture a useful callsite (As UserStackFrame is not guranteed unless another target correctly requests callsite info).

[SergeyKanzhelev]

@snakefoot thanks for another good contribution!

@jeff-zohrab are you around to provide feedback?

[SergeyKanzhelev]

LGTM. Can you please update the branch? Also I'll wait a day to give chance for additional feedback

[snakefoot]

@SergeyKanzhelev Have updated the PR-branch from origin

[SergeyKanzhelev]

@jeff-zohrab thank you for review!