Fix span sub-type missing in breakdown metrics

[SylvainJuge]

Fixes broken breakdown metrics reporting.

For example, when there was multiple database vendors used, only one of them was included in breakdown metrics, thus with an app with mysql and redis, at most one of them was being reported, but not both.

What does this PR do?

• span sub-type was not included in Labels equals/hashcode
• span sub-type was not being properly reset (for mutable labels)

Checklist

• My code follows the style guidelines of this project
• I have rebased my changes on top of the latest master branch
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• I have updated CHANGELOG.asciidoc
• I have updated supported-technologies.asciidoc
• Added an API method or config option? Document in which version this will be introduced
• Added an instrumentation plugin? How did you make sure that old, non-supported versions are not instrumented by accident?

Related issues

• Fixes ElasticAPM: Time spent by span type not displaying correctly when there are multiple databases kibana#59692 (issue initially reported on Kibana)

Screenshot

With an application that has two database spans sub-types: h2 and other

Before

After