Add chain length metrics to gpbft for insight into the quality of proposals

[masih]

We already have metrics for phase transitions. But not for properties of the chain being proposed/propagated. The length of that chain is a key indicator for monitoring purposes:

• length of zero would indicate base
• length of 100 during bootstrap (or when F3 is lagging) would indicate good catchup velocity

..and so on.

Add metrics that expose the distribution of the length of chains recieved/communicated tagged by phase.