Add the Enabled method to the Logger (#5071)

* Add the Enabled method to the Logger

* Add a changelog entry

* Rename enabled.go to min_sev.go

* Remove MinSeverityProcessor

* Document lack of interaction between OnEmit and Enabled

* Update sdk/log/processor.go

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

log/noop/noop.go

@@ -45,3 +45,6 @@ type Logger struct{ embedded.Logger }
 
 // Emit does nothing.
 func (Logger) Emit(context.Context, log.Record) {}
+
+// Enabled returns false. No log records are ever emitted.
+func (Logger) Enabled(context.Context, log.Record) bool { return false }