Add a toggle to disable the HTLC monitor metrics

[reynico]

Over 76k entries are registered for lnd_htlcs_resolved_htlcs on the Prometheus endpoint in less than a week, leading to huge amounts of data being consumed by the Prometheus server (around 10MB per request).

% cat metrics | sort | uniq -c | sort -nr | head -10
65926 lnd_htlcs_resolved_htlcs
8404 lnd_htlcs_resolution_time_bucket
 764 lnd_htlcs_resolution_time_sum
 764 lnd_htlcs_resolution_time_count
  41 lnd_channels_updates_count
  41 lnd_channels_unsettled_balance
  41 lnd_channels_sent_sat
  41 lnd_channels_received_sat
  41 lnd_channels_pending_htlc_count
  41 lnd_channels_fee_per_kw

[guggero]

Thanks for the flag, makes sense to me.

[guggero]

nit: Bad example on the comment above, but we prefer full sentences for comments, including punctuation.

[reynico]

Noted! 497ed32

[guggero]

Sorry, I mostly meant the missing full stop 😅 Feel free to correct that for the existing comment above as well.
And can you please squash the fixup commit with the one that added the line? And usually we prefix the commits with the package that is changed (though not super strictly enforced in this repo): https://github.com/lightningnetwork/lnd/blob/master/docs/code_contribution_guidelines.md

[guggero]

Needs another squash of commits, then this is good to go. Thanks!

[reynico]

Needs another squash of commits, then this is good to go. Thanks!

Awesome! want me to squash the two collectors commits?

[guggero]

Yes, I think everything should go into a single commit.

[Roasbeef]

LGTM 🌊