Disallow duplicate dimension sets.

[jaredcnance]

There is no need for duplicate dimension sets like the following.

{
  "Dimensions": [
    [ "A", "B"],
    [ "A", "B"],
    [ "B", "A"],
  ]
}

This would re-create the same metric 3 times and is equivalent to:

{
  "Dimensions": [ [ "A", "B"] ]
}

This is needed for #14. It allows for things like the following while also allowing for re-use of the logger instance.

const doWork = metricScope(metrics => () => {
  metrics.putDimensions(dimensions);
  // ...
});

// act
doWork();
doWork();

logger.putMetric(metricKey, 0);
await logger.flush();

logger.putMetric(metricKey, 1);
await logger.flush();