Add tx signing metrics

[t-bast]

Monitor the rate at which we sign channel txs and the duration of the signing operations.

[pm47]

The PR is fine, but should we rename the sign methods to signCommitment, signHtlc, signRevoked?

[t-bast]

It's a bit more complicated than that: the metric tags will be slightly inaccurate because some 2nd-level txs (when closing) will be incorrectly assigned to htlc / commit txs.

But the rate of closing txs will be negligible compared to the rate of signing updated commitment txs, so I think the metric can ignore this subtlety, but renaming the methods could be confusing for readers when studying the closing flows.

[codecov-io]

Codecov Report

Merging #1659 (11e5478) into master (dd8975a) will increase coverage by 0.09%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1659      +/-   ##
==========================================
+ Coverage   85.92%   86.01%   +0.09%     
==========================================
  Files         151      151              
  Lines       11471    11487      +16     
  Branches      505      477      -28     
==========================================
+ Hits         9856     9881      +25     
+ Misses       1615     1606       -9     

Impacted Files	Coverage Δ
...q/eclair/crypto/keymanager/ChannelKeyManager.scala	87.50% <ø> (ø)
...main/scala/fr/acinq/eclair/crypto/Monitoring.scala	100.00% <100.00%> (ø)
...air/crypto/keymanager/LocalChannelKeyManager.scala	88.63% <100.00%> (+2.92%)	⬆️
...c/main/scala/fr/acinq/eclair/channel/Channel.scala	86.06% <0.00%> (+0.08%)	⬆️
...-core/src/main/scala/fr/acinq/eclair/io/Peer.scala	89.67% <0.00%> (+1.08%)	⬆️
...main/scala/fr/acinq/eclair/router/Validation.scala	92.91% <0.00%> (+1.49%)	⬆️
...cala/fr/acinq/eclair/payment/relay/NodeRelay.scala	95.27% <0.00%> (+1.57%)	⬆️