Introduce generic metric poll thread interceptor & generic sink

The Spark runner provides a convenient feature which is the metric sink.

By configuration, it allows us to configure a metric sink using metrics.properties configuration containing:

driver.sink.graphite.class=org.apache.beam.runners.spark.metrics.sink.GraphiteSink
driver.sink.graphite.host=localhost
driver.sink.graphite.port=2003
driver.sink.graphite.prefix=spark
driver.sink.graphite.period=1
driver.sink.graphite.unit=SECONDS 

This approach is very convenient to send the metric to any sink. I think we can apply this logic in generic way working with any runner.

The idea is to use a metric poll thread in the pipeline (that we can enable via PipelineOptions) and send to a sink.

I started a PoC about that.