Fixes a bug that would cause the healthcheck service not to properly
check blocks when the target is advancing faster than the reference
client. RHC will now check the block correpsonding to the lesser of the
two block heights. Also adds a new metric to track the height difference
in the two providers which could be used to alert if the target leads
the reference by too much or vice versa.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>