Motivation:
Review code styles with some bots automatically.
Modifications:
Set up ReviewDog in GitHub Action to review code style.
Add more check rules to checkstyle plugin.
Result:
Obvious code styles can be reviewed and commented automatically.
### Motivation:
Reduce footprint when tagging spans with the deprecated API: `AbstractSpan#tag(String, String)`.
### Modifications:
- Adopt [prototype pattern](https://en.wikipedia.org/wiki/Prototype_pattern) to create `Tag`, prevent from creating too many `StringTag` instances and `GC`ed.
- Replace `AbstractSpan#tag(String, String)` with `AbstractSpan#tag(AbstractTag, String)`.
### Result:
- Footprint is reduced.
吴晟 Wu Sheng
kezhenxu94
于玉桔