Log suppressed exceptions in MillisecondPrecisionSyslogAppender

[totalorder]

This adds support for logging suppressed exceptions. Without this change they are silently ignored contrary to a regular throwable.printStackTrace() which does print them.
Inspiration has been taken from the upstream implementation in another appender: https://github.com/qos-ch/logback/blob/master/logback-classic/src/main/java/ch/qos/logback/classic/pattern/RootCauseFirstThrowableProxyConverter.java#L33

[totalorder]

Is there a reasonably easy way to write tests for this class? I don't even know where to start.
In the mean time we are testing this by shadowing it in our service. Testing in production 🤠 https://ghe.spotify.net/search-platform/search-api/pull/1116

[totalorder]

@danielnorberg @pettermahlen Please take a look

[pettermahlen]

LGTM, perhaps give the shadow testing a couple of days before merge?

[totalorder]

Sounds good! We'll run it in production for a few days before merge. I want to make sure google stackdriver manages to group the stacktrace correctly too.

[totalorder]

@pettermahlen We have been running this in production for ~10 days and it seems to work well. Would you mind merging it?

[totalorder]

@pettermahlen Thanks! I saw the last release was ~1 year ago. Is there still someone around who knows how to make a new release of this package? :)

[pettermahlen]

@totalorder I will dare it, any time now :) Not sure that the current laptop is set up for it, but we'll see. Not sure I'll get to it today, a lot of meetings.

[pettermahlen]

2.2.7 is now out and available via Maven Central.

[totalorder]

Awesome! You are a brave soul