Received metrics are not thread-safe

In the logs we often see stuff like:

2012-09-28 16:47:28,835 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 9.2083, MBytesSent = 0.0215, MBytesReceived = 283.1398, ave sent request MBytes = 0, ave received request MBytes = 0.0762, secs waited = 30.886
2012-09-28 16:47:28,835 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 9.1614, MBytesSent = 0.0215, MBytesReceived = 281.83, ave sent request MBytes = 0, ave received request MBytes = 0.076, secs waited = 0.0
2012-09-28 16:47:28,835 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 0, MBytesSent = 0.0215, MBytesReceived = 281.6554, ave sent request MBytes = 0, ave received request MBytes = 0.076, secs waited = 0.0
2012-09-28 16:47:28,835 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 0, MBytesSent = 0.0215, MBytesReceived = 282.3169, ave sent request MBytes = 0, ave received request MBytes = 0.0761, secs waited = 0.0
2012-09-28 16:47:28,836 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 0, MBytesSent = 0.0215, MBytesReceived = 282.3169, ave sent request MBytes = 0, ave received request MBytes = 0.0761, secs waited = 0.001
2012-09-28 16:47:28,836 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 144.7954, MBytesSent = 0.0215, MBytesReceived = 282.6441, ave sent request MBytes = 0, ave received request MBytes = 0.0761, secs waited = 0.0
2012-09-28 16:47:28,836 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 9.1406, MBytesSent = 0.0215, MBytesReceived = 281.4949, ave sent request MBytes = 0, ave received request MBytes = 0.0759, secs waited = 0.0
2012-09-28 16:47:28,836 INFO org.apache.giraph.comm.netty.handler.RequestDecoder: decode: Server window metrics MBytes/sec sent = 0.0007, MBytes/sec received = 9.1673, MBytesSent = 0.0215, MBytesReceived = 282.6441, ave sent request MBytes = 0, ave received request MBytes = 0.0761, secs waited = 0.0

This happens due to multiple threads calling ByteCounter#getMetricsWindow() and the method not being thread safe.