Add code examples to the 'tags' package.
See https://github.com/census-instrumentation/opencensus-java/pull/485#pullrequestreview-53065563 for more suggestions on documentation for tags.
This issue is for adding examples to Javadocs in package-info.java and classes like TagContextBuilder or Tagger.
Duplicated of https://github.com/census-instrumentation/opencensus-java/issues/1246.
This issue is about improving the Javadocs for the tagging, for example, TagContextBuilder and Tagger. I think the examples can be much simpler now that there aren't multiple key types, though.
Oh I see, I was thinking this issue is only about package-info.java. Reopened.