Add a log message during an epoch transition

[Kay-Zee]

Example log if we want one line per epoch transition.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 41.50%. Comparing base (5be1172) to head (ffd69a5).

Additional details and impacted files

@@            Coverage Diff             @@
##            v0.37    #6470      +/-   ##
==========================================
- Coverage   41.51%   41.50%   -0.01%     
==========================================
  Files        2017     2017              
  Lines      144007   144013       +6     
==========================================
- Hits        59778    59771       -7     
- Misses      78028    78039      +11     
- Partials     6201     6203       +2     

Flag	Coverage Δ
unittests	41.50% <100.00%> (-0.01%)	⬇️

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[jordanschalm]

Looks good, depending on what we want. Some notes:

• This will not log the first height for epochs beginning after a spork
  • if people are using these height values primarily to check the reward payout events, then the height we record for post-spork epochs in this way would not contain the reward payouts anyway
  • for post-spork epochs, the height for getting rewards would be the height of the resetEpoch transaction
• We actually do have a metric tracking the first height of the current epoch: consensus_compliance_last_epoch_transition_height. It's a gauge that increases at every epoch transition (it also doesn't seem to handle post-spork epochs properly, though...).

[jordanschalm]

Suggested change

	Uint64("new_epoch_height", finalized.Height).
	Uint64("new_epoch_first_height", finalized.Height).