Add more logs and round members metric

[poszu]

Closes #336

• added some logs, especially log the number of round members when it's recovered and started
• refactored existing logs
• improved logging around context cancellation (i.e. previously it incorrectly logged round failure when service was stopped)
• added metric showing the number of members
• refactored leaves counter metric

  # HELP poet_round_leaves_total Number of generated leaves in a round
  # TYPE poet_round_leaves_total counter
  poet_round_leaves_total{ID="2"} 1.478935e+06
  poet_round_leaves_total{ID="3"} 0
  # HELP poet_round_members_total Number of members in a round
  # TYPE poet_round_members_total counter
  poet_round_members_total{ID="2"} 0
  poet_round_members_total{ID="3"} 1
  

[codecov]

Codecov Report

Merging #338 (89ee249) into develop (e90b082) will increase coverage by 0.1%.
Report is 2 commits behind head on develop.
The diff coverage is 73.1%.

@@            Coverage Diff            @@
##           develop    #338     +/-   ##
=========================================
+ Coverage     73.0%   73.1%   +0.1%     
=========================================
  Files           17      17             
  Lines         1497    1561     +64     
=========================================
+ Hits          1093    1142     +49     
- Misses         298     315     +17     
+ Partials       106     104      -2     

Files Changed	Coverage Δ
server/server.go	74.0% <33.3%> (-0.6%)	⬇️
service/service.go	76.2% <44.6%> (-2.7%)	⬇️
service/round.go	78.8% <92.6%> (+5.6%)	⬆️
prover/prover.go	59.5% <100.0%> (-0.2%)	⬇️

... and 1 file with indirect coverage changes