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

[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.