code: public <T> AsyncMutateInBuilder arrayInsert(String path, T value) {
          if (StringUtil.isNullOrEmpty(path)) {
              throw new IllegalArgumentException("Path must not be empty for arrayInsert");
          }
          this.mutationSpecs.add(new MutationSpec(Mutation.ARRAY_INSERT, path, value));
          return this;
      }
docs: Insert into an existing array at a specific position
  (denoted in the path, eg. "sub.array[2]").
  
  @param path the path (including array position) where to insert the value.
  @param value the value to insert in the array.