Contributes to #2054

Changes

Performed general cleanup to resolve issues reported by include-what-you-use:

• Cleanup in SDK for common
• Cleanup in SDK for traces
• Cleanup in SDK for metrics
• Cleanup in SDK for logs
• Cleanup in the ostream exporter
• Cleanup in the memory exporter
• Cleanup in examples

For the most part, applying the code suggestions from include-what-you-use was sufficient.

In some cases, in particular for:

namespace nostd = opentelemetry::nostd;

nostd::xxx foo;

the namespace directive causes iwyu to want a header to declare the nostd namespace itself.

Because there are many available headers, iwyu does not suggest the best choice.

Resolved by removing the namespace alias, and use fully qualified names instead:

opentelemetry::nostd::xxx foo;

Note that further cleanup is required, for code build optionally (various exporters, unit tests, etc).

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed