Support metric rollup for different dimensionset values

Currently, if a metric has dimensions and customers need to create alarms on metrics aggregated across all the values of the dimensions, customers need to emit the same metric with different combination of the dimensionset.

Example:
1. DimensionSet: [d1, d2, d3]
2. Possible Dimension Values: d1:[d1_1, d1_2]; d2:[d2_1, d2_2]; d3:[d3_1, d3_2]

If user wants to create alarms on all values of [d1, d2, d3], they need to add all combinations to the MetricsLogger.
[[d1, d2, d3],[d1, d2],[d1, d3],[d2, d3],[d1],[d2],[d3],[]]

Request here is to provide a mechanism in this library to automatically populate all combinations for the dimensions.

Example:
metricsLogger.putDimensionSetCombination(DimensionSet.of(d1, d1_1, d2, d2_2, d3, d3_1)) should add [[d1, d2, d3],[d1, d2],[d1, d3],[d2, d3],[d1],[d2],[d3],[]] as the dimensionset in the log.