Add watchtower client number of backup metrics

[Jontified]

Summary: This PR adds support for tracking the number of backups that the watchtower client has recorded.

Purpose: We are interested in keeping track of if our watchtower is functioning and making sure that the number of backups corresponds to the number of transactions is one metric we would like to use for this purpose.

Code:
This PR adds a new collector, wt_client_collector.go, which works very similarly to previous collectors.
This PR relies on an updated version of lndclient (update here) to communicate with the watchtower client interface of lnd.

Semantics:
2 metrics are exposed num_backups and num_pending_backups.
The metrics will not show up at all if there is no tower active.
Each metric has a towers public key as a label.

[Jontified]

This will have to wait for lightninglabs/lndclient#235 to be merged before the proper lndclient dependency can be added in go.mod

[bitromortac]

Looks good, thanks for the addition 🗼! Could you make sure to change commit message headers to be consistent with the repo's (just have a look at the past ones). For example Update go.mod and go.sum would become mod: update go.mod and go.sum or Fix comment would be collectors: fix comment (i.e. prefixing with the system/topic it concerns).

We also don't have a long line linter active in this repo, please try to cap lines at 80 chars. Will do a test run after your fixes so that we can merge the lndclient PR and then update the library in here.

[Jontified]

Things should be up to date now!

[bitromortac]

Looking great, we need to catch errors if the watchtower client wasn't enabled, see the comment, otherwise lndmon won't run. Please make sure to run the linter before you push, the code requires reformatting. I have tested backup metrics collection as well 🎉

[Jontified]

Final look and final comments?

[bitromortac]

Tested again, LGTM 🎉. Thanks again!

A next step would be to add a dashboard such that other users will have access to those numbers by default (if you like to do another project).

[hieblmi]

Thanks for your effort, I just flagged some minor improvements, otherwise looks good to go.

[hieblmi]

Great work, LGTM✅