rename MetricsCollection.getMetrics() to toMetrics().

This makes it compatible with MetricsContainer.
Make MetricsContainer an interface.
Change MetricsContainer/MetricsCollection to use Collection in exposed methods.
Obviate MetricsContainer
MetricsCollection houses IMetric instead of Metric.

There were a few casualties:

* In a few tests, duplicate collections were needed to create Collection<IMetric> from Collection<Metric>
* IMetric has a new setTtlInSeconds method.
* IncomingMetricMetadataAnalyzer checks instance before scanning, so it only operates on Metric.
Merge pull request #217 from gdusbabek/metrics_container_collection

Refine MetricsContainer and MetricsCollection interfaces.