NR-64056 Error grouping

[jtduffy]

Implements the error grouping feature:

• Callback handler and error data classes
• API upgrade for callback registration
• Callback invoked for each error reported to NR
• Javadocs
• Associated unit tests

[obenkenobi]

Overall this looks good. There are some NPE risks and minor code style comments I'd like to be addressed beforehand.

[meiao]

I'd avoid the manual stack trace to string code there. You could use

NewRelic.getAgent().getLogger().log(Level.FINEST, e, "Customer errorGroupCallback stacktrace.");

though it will repeat the message.

So another option would be:

if (NewRelic.getAgent().getLogger().isLoggable(Level.FINEST)) {
  NewRelic.getAgent().getLogger().log(Level.FINEST, e, "Customer errorGroupCallback threw an exception.");
} else {
  NewRelic.getAgent().getLogger().log(Level.WARNING, "Customer errorGroupCallback implementation threw an exception: {0}", e.getMessage());
}