Ensures that span is closed when Observable is unsubscribed from.
Also added retransform error logger since retransforms might be missed if an exception is thrown, leading to odd behavior.
Add the following system property or corresponding environment variable:
```
dd.trace.classes.exclude=some.packagename.*,some.classname.MyClass$NestedClass
```
It turns out that Eclipse's OSGi implementation has two problems:
* It doesn't respect system properties by default
* It has tricky classloader implementation that loads bootstrap
classes from the classloader-has-delegation-to-bootstrap check but
doesn't load bootstrap classes from 'normal' code.
This should address second problem and make Eclipse's OSGi
implementation 'safe' by default.
Laplie Anderson
Luca Abbati
Tyler Benson
Nikolay Martynov
Andrew Kent