make PatternUtils.escape public (#994)

Can be useful for more precisely mapping fixed strings
to a regex pattern.

spectator-api/src/main/java/com/netflix/spectator/impl/matcher/PatternUtils.java

@@ -160,7 +160,7 @@ static String expandEscapedChars(String str) {
    * as a regular expression.
    */
   @SuppressWarnings("PMD.NcssCount")
-  static String escape(String str) {
+  public static String escape(String str) {
     StringBuilder builder = new StringBuilder();
     for (int i = 0; i < str.length(); ++i) {
       char c = str.charAt(i);