[ETW] ETW log exporter possible memory ownership issues

[marcalff]

This is not confirmed with testing, only a suspicion raised by code review.

The ostream log exporter has memory ownership issues that can lead to crashes.

See related:

• #3135
• #2651
• #3417
• #3147

It turns out (seen by code review) that file etw_logger.h uses exactly the same pattern, so it should be subject to the same issues.

  std::unordered_map<std::string, opentelemetry::common::AttributeValue> attributes_map_;
  opentelemetry::common::AttributeValue body_ = opentelemetry::nostd::string_view();

attributes_map_ and body_ can contain stale pointers.