code: public void increment( ValueMetric metric,
                      long incrementalValue ) {
          assert metric != null;
          ValueHistory history = values.get(metric);
          if (history != null) history.recordIncrement(incrementalValue);
      }
docs: Record an incremental change to a value, called by the code that knows when and how the metric changes.
  
  @param metric the metric; may not be null
  @param incrementalValue the positive or negative increment
  @see #increment(ValueMetric)
  @see #decrement(ValueMetric)
  @see #recordDuration(DurationMetric, long, TimeUnit, Map)