metrics: add expected height metric

[travisperson]

Exports the expected height of the node based off of the current time, block delay and the genesis timestamp.

Between the following stats we should be able to get some pretty good information about the status of a node with regard to it's chain

# HELP lotus_chain_node_height Current Height of the node
# TYPE lotus_chain_node_height gauge
lotus_chain_node_height 35354
# HELP lotus_chain_node_height_expected Expected Height of the node
# TYPE lotus_chain_node_height_expected gauge
lotus_chain_node_height_expected 35354
# HELP lotus_chain_node_worker_height Current Height of workers on the node
# TYPE lotus_chain_node_worker_height gauge
lotus_chain_node_worker_height 35354

[magik6k]

Couple of nits, really like the idea though