fix: retain level when using WithAttrs/WithGroup

Because these methods were not persisting the level in the GoKitHandler
through to the new handler, the level would be unset in later log calls,
resulting in the level dropping back to info by default.

Signed-off-by: TJ Hoplock <[email protected]>

handler.go

@@ -87,7 +87,12 @@ func (h *GoKitHandler) WithAttrs(attrs []slog.Attr) slog.Handler {
 		pairs = append(h.preformatted, pairs...)
 	}
 
-	return &GoKitHandler{logger: h.logger, preformatted: pairs, group: h.group}
+	return &GoKitHandler{
+		logger:       h.logger,
+		level:        h.level,
+		preformatted: pairs,
+		group:        h.group,
+	}
 }
 
 func (h *GoKitHandler) WithGroup(name string) slog.Handler {
@@ -100,7 +105,12 @@ func (h *GoKitHandler) WithGroup(name string) slog.Handler {
 		g = h.group + "." + g
 	}
 
-	return &GoKitHandler{logger: h.logger, preformatted: h.preformatted, group: g}
+	return &GoKitHandler{
+		logger:       h.logger,
+		level:        h.level,
+		preformatted: h.preformatted,
+		group:        g,
+	}
 }
 
 func appendPair(pairs []any, groupPrefix string, attr slog.Attr) []any {