code: public static MountContext mergeFrom(MountPOptions.Builder optionsBuilder) {
      MountPOptions masterOptions = FileSystemOptions.mountDefaults(ServerConfiguration.global());
      MountPOptions.Builder mergedOptionsBuilder =
          masterOptions.toBuilder().mergeFrom(optionsBuilder.build());
      return create(mergedOptionsBuilder);
    }
docs: Merges and embeds the given {@link MountPOptions} with the corresponding master options.
  
  @param optionsBuilder Builder for proto {@link MountPOptions} to embed
  @return the instance of {@link MountContext} with default values for master