add Strings#isDigits API

inspiration taken from [this SO answer][SO].

note that the steram is not parallelised to avoid the overhead of this
as the method is intended to be called primarily with shorter strings
where the time to set up would take longer than the actual check.

[SO]: https://stackoverflow.com/a/35150400

Signed-off-by: Ralph Ursprung <Ralph.Ursprung@avaloq.com>

libs/core/src/main/java/org/opensearch/core/common/Strings.java

@@ -815,4 +815,17 @@ public static String toLowercaseAscii(String in) {
         }
         return out.toString();
     }
+
+    /**
+     * Check whether every single character in the string is a digit.
+     *
+     * Important: this also returns {@code true} for empty strings!
+     *
+     * @param s the string, must not be null.
+     * @return true if the string is empty or only contains digits, false otherwise.
+     */
+    public static boolean isDigits(final String s) {
+        return s.chars().allMatch(Character::isDigit);
+    }
+
 }

libs/core/src/test/java/org/opensearch/core/common/StringsTests.java

@@ -114,4 +114,14 @@ public void testToStringToXContentWithOrWithoutParams() {
             containsString("\"color_from_param\":\"blue\"")
         );
     }
+
+    public void testIsDigits() {
+        assertTrue(Strings.isDigits(""));
+        assertTrue(Strings.isDigits("123"));
+        assertFalse(Strings.isDigits("abc"));
+        assertFalse(Strings.isDigits("123a"));
+        assertFalse(Strings.isDigits("0x123"));
+        assertFalse(Strings.isDigits("123.4"));
+        assertFalse(Strings.isDigits("123f"));
+    }
 }