Make statsd sample rate behave better.

As Dieter pointed out in bug 1090495
(https://bugs.launchpad.net/swift/+bug/1090495), the volume of metrics
can vary wildly between StatsD metrics.

This patch implements a partial solution by reducing the sample_rate
used for known high-volume metrics (operational experience will need to
inform this over time) and introducing a new tunable,
log_statsd_sample_rate_factor which is multiplied by the sample_rate for
every statsd stat.  This tunable can be used to reduce StatsD traffic
proportionally for all metrics and is intended to replace
log_statsd_default_sample_rate, which is left alone for
backward-compatibility, should anyone be using it.

This patch also includes a drive-by fix for log_udp_port which wasn't
being converted to an int (I didn't verify that actually causes trouble
in SysLogHandler(), but it's definitely an improvement regardles).

Change-Id: Id404636e3629f6431cf1c4e64a143959750a3c23
Merge "Make statsd sample rate behave better."