feat: tracks block time using a Prometheus gauge metric type

[staheri14]

Closes #1077

I have verified the outcome of this pull request, and the block time gauge now correctly appears in the Prometheus endpoint:

# HELP cometbft_consensus_block_time_seconds Duration between this block and the preceding one.
# TYPE cometbft_consensus_block_time_seconds gauge
cometbft_consensus_block_time_seconds{chain_id="mocha-4",version="1.0.0-rc16"} 11.623671263

[staheri14]

I've decided to remove the block height label from the block_time_seconds gauge metric. This decision is primarily due to concerns related to increased metric cardinality. Including block height as a label would result in a linear increase in the number of unique label values, which could potentially lead to storage issues and slower Prometheus query performance.

[evan-forbes]

👍