Appender-Tracing - handle target and scope

[cijothomas]

#1683 this stored target in a top level field and have OTLP Exporter treat it as scope.

The tracing-appender information is totally lost now.
open-telemetry/semantic-conventions#1550 proposes to store this in scope attributes. Need to follow up.

Additionally, register_callsite may be overridden to get the target name once for every callsite instead of every invocation. This may be leveraged to maintain a Map of Loggers per target. (This will cause contention, or need to use thread-local map!)