add an optionals utility and fix push challenge metric

service/src/main/java/org/whispersystems/textsecuregcm/util/Optionals.java

@@ -0,0 +1,21 @@
+package org.whispersystems.textsecuregcm.util;
+
+import java.util.Optional;
+import java.util.function.BiFunction;
+
+public class Optionals {
+
+  private Optionals() {}
+
+  /**
+   * Apply a function to two optional arguments, returning empty if either argument is empty
+   *
+   * @param optionalT Optional of type T
+   * @param optionalU Optional of type U
+   * @param fun       Function of T and U that returns R
+   * @return The function applied to the values of optionalT and optionalU, or empty
+   */
+  public static <T, U, R> Optional<R> zipWith(Optional<T> optionalT, Optional<U> optionalU, BiFunction<T, U, R> fun) {
+    return optionalT.flatMap(t -> optionalU.map(u -> fun.apply(t, u)));
+  }
+}