Clean-up of ETW exporter

The following feature may be reusable by other components:

• Owning Properties collection (container) with convenient initializer list operators and constructors to pass incoming event properties as std::string, vectors of integers/floats, string literals, etc. The way how it should be to be simple and look like JSON object initialization.

Usage example

// Span attributes
  Properties attribs =
  {
    {"attrib1", 1},
    {"attrib2", 2}
  };
  auto outerSpan = tracer->StartSpan("MySpanOuter", attribs);

  // Add first event
  std::string eventName1 = "MyEvent1";
  Properties event1 =
  {
    {"uint32Key", (uint32_t)1234},
    {"uint64Key", (uint64_t)1234567890},
    {"strKey", "someValue"}
  };
  outerSpan->AddEvent(eventName1, event1);

  // Add second event
  std::string eventName2 = "MyEvent2";
  Properties event2 =
  {
    {"uint32Key", (uint32_t)9876},
    {"uint64Key", (uint64_t)987654321},
    {"strKey", "anotherValue"}
  };
  outerSpan->AddEvent(eventName2, event2);

  // Create nested span. Note how we share the attributes here..
  // It is Okay to either share, OR have your own attributes map.
  auto innerSpan = tracer->StartSpan("MySpanInner", attribs);
  std::string eventName3= "MyEvent3";
  Properties event3 =
  {
    {"uint32Key", (uint32_t)9876},
    {"uint64Key", (uint64_t)987654321},
    // {"int32array", {{-1,0,1,2,3}} },
    // Since this collection is owning, we may store values returned by other utility functions
    {"tempString", getTemporaryValue() }
  };
  
  innerSpan->AddEvent(eventName3, event3);
  innerSpan->End();    // end innerSpan
  outerSpan->End();    // end outerSpan
  tracer->CloseWithMicroseconds(0);

Note that the new class is fully compatible with KeyValueIterable, i.e. provides ABI-safe transform - should there be the need for ABI-safe interface. Note that across Visual Studio 2015-2017-2019, the ABI-safety for STL types is readily guaranteed on Windows. Thus, strictly speaking, unnecessary requirement that can be heavily optimized by avoiding the unnecessary tranformation.

ETW channel -specific improvements

List of features added:

• preliminary support for Span attributes and links
• adding TraceId + SpanId on individual events
• separate expression for Spans being started / stopped; support for nested Spans
• code comments clean-up
• code refactor

Tests

ETW listener 'Test Framework' is written in C# - will be submited in contrib repo. This code also provides ability to Debug OpenTelemetry ETW exporter events in Visual Studio 2019 using built-in Diagnostic Events tab. Code is vendor-agnostic. I included examples of some other vendors using ETW. Those could be the candidates to adopt OpenTelemetry C++ SDK with ETW channel. Field names for ETW channel are also configurable, i.e. vendors may decide their own naming convention. It would make sense to normalize this with OTLP protocol names.

Debugging Telemetry in Visual Studio 2019

Example screenshot that illustrates how test events show-up in Visual Studio 2019 with no additional tooling necessary:

This article explains more about Event Tracing for Windows and possible ways to export the data further:
https://docs.microsoft.com/en-us/azure/service-fabric/service-fabric-diagnostics-how-to-monitor-and-diagnose-services-locally

Possible flows will be explained further in contrib repo. Existing tools that allow to monitor and export ETW events to Elastic Search, for example: https://www.fireeye.com/blog/threat-research/2019/03/silketw-because-free-telemetry-is-free.html