add head metrics gauge

[garyschulte]

PR Description

Add a metrics gauge tracking shomei head state: shomei_blockchain_head
e.g.:

$ curl -s localhost:9888/metrics|grep shomei
# HELP shomei_blockchain_head current chain head block number which corresponds to shomei state
# TYPE shomei_blockchain_head gauge
shomei_blockchain_head 257373.0


$ curl -s localhost:9888/metrics|grep shomei
# HELP shomei_blockchain_head current chain head block number which corresponds to shomei state
# TYPE shomei_blockchain_head gauge
shomei_blockchain_head 257642.0

also removes the expectation that a MetricsService is a vertx verticle..

Fixed Issue(s)

#99

Documentation

• I thought about documentation and added the doc-change-required label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.