Support for custom metrics

Currently Quarks provides the following metrics encapsulated in Oplet implementations:

• tuple counter (number of tuples since topology submission)
• tuple meter (mean throughput and moving averages).

Additional support is needed to satisfy the following usage scenarios:

• Using metrics in lambda functions: a developer writes a function using Java metrics directly.
• Define oplet which encapsulates multiple metrics
• Metric naming: a logical name is assigned to a metric when a topology is declared; name is used to access the metric at run time.

For reference the initial discussion was added under this issue: https://github.com/quarks-edge/quarks/issues/117