Add comments clarifying why promql.Querylogger exists (#17231)

And why we only have one implementation in this code base.

Fixes: https://github.com/prometheus/prometheus/issues/15869

Signed-off-by: Jan Fajerski <jfajersk@redhat.com>

promql/engine.go

@@ -125,6 +125,8 @@ var _ QueryLogger = (*logging.JSONFileLogger)(nil)
 
 // QueryLogger is an interface that can be used to log all the queries logged
 // by the engine.
+// logging.JSONFileLogger implements this interface, downstream users may use
+// different implementations.
 type QueryLogger interface {
 	slog.Handler
 	io.Closer

util/logging/file.go

@@ -27,8 +27,8 @@ var _ slog.Handler = (*JSONFileLogger)(nil)
 
 var _ io.Closer = (*JSONFileLogger)(nil)
 
-// JSONFileLogger represents a logger that writes JSON to a file. It implements
-// the slog.Handler interface, as well as the io.Closer interface.
+// JSONFileLogger represents a logger that writes JSON to a file.
+// It implements the promql.QueryLogger interface.
 type JSONFileLogger struct {
 	handler slog.Handler
 	file    *os.File