Update to AutoTrace (dotnet/coreclr#24936)

* * Add license headers to autotrace.h|cpp
* use W() macro for getting correct string literal type
* formalize env vars to be COMPlus_* style
* add documentation
* modify cmake files to have the flag and set default value to 0

* Fix typo


Commit migrated from b3ff5135ec

docs/coreclr/workflow/AutomatedStressTestingForDiagnosticServer.md

@@ -0,0 +1,23 @@
+#AutoTrace: 
+
+> see: `src/vm/autotrace.h|cpp` for the code
+
+AutoTrace is used to run automated testing of the Diagnostic Server based tracing and specifically
+EventPipe.  The feature itself is enabled via the feature flag `-DFEATURE_AUTO_TRACE`.
+
+## Mechanism:
+
+AutoTrace injects a waitable event into the startup path of the runtime and waits on that event until
+some number of Diagnostics IPC (see: Diagnostics IPC in the dotnet/diagnostics repo) connections have occurred.
+The runtime then creates some number of processes using a supplied path that typically are Diagnostics IPC based tracers.
+Once all the tracers have connected to the server, the event will be signaled and execution will continue as normal.
+
+## Use:
+
+Two environment variables dictate behavior:
+- `COMPlus_AutoTrace_N_Tracers`: The number of tracers to create.  Should be a number in `[0,64]` where `0` will bypass the wait for attach.
+- `COMPlus_AutoTrace_Command`: The path to the executable to be invoked.  Typically this will be a `run.sh|cmd` script.
+
+> (NB: you should `cd` into the directory you intend to execute `COMPlus_AutoTrace_Command` from as the first line of the script.)
+
+Once turned on, AutoTrace will run the specified command `COMPlus_AutoTrace_N_Tracers` times.