Add metrics on splicing

[pm47]

This is best effort but should already be pretty useful.

Please double check the attribution logic, as it's not trivial. 🙏

[codecov-commenter]

Codecov Report

Merging #2756 (b58a3bd) into master (e3ba524) will increase coverage by 0.07%. Report is 5 commits behind head on master. The diff coverage is 98.40%.

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@@            Coverage Diff             @@
##           master    #2756      +/-   ##
==========================================
+ Coverage   85.75%   85.83%   +0.07%     
==========================================
  Files         216      216              
  Lines       18061    18089      +28     
  Branches      743      738       -5     
==========================================
+ Hits        15488    15526      +38     
+ Misses       2573     2563      -10     

Files	Coverage Δ
...re/src/main/scala/fr/acinq/eclair/NodeParams.scala	93.37% <100.00%> (+0.04%)	:arrow_up:
...r/acinq/eclair/blockchain/fee/OnChainFeeConf.scala	95.00% <ø> (-0.24%)	:arrow_down:
...cala/fr/acinq/eclair/channel/ChannelFeatures.scala	100.00% <100.00%> (ø)
...in/scala/fr/acinq/eclair/channel/Commitments.scala	96.82% <100.00%> (+<0.01%)	:arrow_up:
...c/main/scala/fr/acinq/eclair/channel/Helpers.scala	94.51% <100.00%> (+0.02%)	:arrow_up:
...in/scala/fr/acinq/eclair/channel/fsm/Channel.scala	85.94% <100.00%> (+0.64%)	:arrow_up:
...q/eclair/channel/fsm/ChannelOpenSingleFunded.scala	93.46% <100.00%> (ø)
...inq/eclair/channel/fund/InteractiveTxBuilder.scala	91.47% <100.00%> (+0.10%)	:arrow_up:
...cinq/eclair/channel/fund/InteractiveTxFunder.scala	92.18% <100.00%> (ø)
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	87.60% <100.00%> (ø)
... and 3 more

... and 6 files with indirect coverage changes