Logging splits up multiline log messages

Log entries spanning multiple lines like stack traces are split up and end up as multiple oneline entries in the Logs Explorer. Fluentbit provides [the “Multiline” filter](https://docs.fluentbit.io/manual/pipeline/filters/multiline-stacktrace) to deal with that, so I propose to implement it and add a `filters` element to the configuration model.

This is not a duplicate of #239: This issue requests support of GCP Cloud Tracing whereas I’m asking for proper treatment of stack traces for GCP Logging.