ETW example and tests clean-up

[maxgolov]

Changes

Customer ask:

• add example of a multi-threaded application. Illustrate best practices on passing context from main thread to workers.
• added document that explains the flow and usage instructions how to debug the flow in Visual Studio using ETW exporter.

Clean-up:

• fix typo
• do not pass 2nd argument in ETW tests. It is misleading (should be version), and unused by ETW header-only tracer anyways.

Additional tests:

• illustrate how to create global tracer provider using magic statics.
• obtain different tracers, verify that they get different trace id.
• verify that the same global tracer returns the same trace id.

Also I added some comments showing event payload contents in ETW tests.
This comes in handy to better understand how provider works.

No changes to public API.

[codecov]

Codecov Report

Merging #882 (b880e30) into main (b5dd914) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #882   +/-   ##
=======================================
  Coverage   95.44%   95.44%           
=======================================
  Files         157      157           
  Lines        6633     6633           
=======================================
  Hits         6330     6330           
  Misses        303      303           

Impacted Files	Coverage Δ
...pi/include/opentelemetry/context/runtime_context.h	97.47% <ø> (ø)