code: @Nullable
      public <T extends Number> T gauge(String name, T number) {
          return gauge(name, emptyList(), number);
      }
docs: Register a gauge that reports the value of the {@link Number}.
  
  @param name   Name of the gauge being registered.
  @param number Thread-safe implementation of {@link Number} used to access the value.
  @param <T>    The type of the state object from which the gauge value is extracted.
  @return The number that was passed in so the registration can be done as part of an assignment
  statement.