Collect more metrics from Siphon

This is open to discussion, I was thinking about tracking the following metrics (all metrics are per relname):

• Actual buffer size (number of items) before serialization.
• Serialization speed.
• Serialized package size.
• Compression speed.
• Compressed package size.
• Compression ratio.
• Queueing time (how long does it take to push the package into the queue).