Remove braces from suggestion

This commit removes the open and close braces from the suggestion
as braces do not make sense in the configuration file. This does
not change the behavior of the suggestion whatsoever as these
are optional.

matchers/compat/parse.go

@@ -152,14 +152,12 @@ func fallbackMatchersParser(l log.Logger) matchersParser {
 				return nil, invalidErr
 			}
 			var sb strings.Builder
-			sb.WriteRune('{')
 			for i, n := range m {
 				sb.WriteString(n.String())
 				if i < len(m)-1 {
 					sb.WriteRune(',')
 				}
 			}
-			sb.WriteRune('}')
 			suggestion := sb.String()
 			// The input is valid in the old pkg/labels parser, but not the
 			// new matchers/parse parser.