Better support for Semantic Logger

[unflxw]

Currently, our documentation says to use Semantic Logger with AppSignal by passing our logger to Semantic Logger, and using the Logfmt formatters on both Semantic Logger's appender and our own logger.

However, going by customer reports (Intercom link) this does not result on a good experience. Semantic Logger adds lots of attributes that are not particularly useful. The custom attributes added by the customer appear later in the Logfmt-formatted log line, which causes them to be cut off by our logging system's attribute limit.

We could probably improve on this by writing a Semantic Logger appender, rather than using the generic Semantic Logger appender for Logger instances.