[metrics] - Block Height

[jbearer]

What is this task and why do we need to work on it?

#1564

We provide block height in its own endpoint in the query service, but Blockdaemon has requested that we also include it in the Prometheus metrics, which I think makes it easier for them to implement monitoring.

What work will need to be done to complete this task?

• Add a new counter to the consensus metrics
• Increment the counter at each decide

Are there any other details to include?

No response

What are the acceptance criteria to close this issue?

• A new block_height counter metrics exists, and always reflects the number of decided leaves

[dailinsubjam]

Done in #1852.