collectors: collect payment and attempt counts

[calvinrzachman]

Add support for real time tracking of counters for payments and attempts. These can be used to compute and create visualizations for things like average payment throughput and average # of attempts per payment.

rate(lnd_total_payments[5m])
rate(lnd_total_htlc_attempts[30m]) / rate(lnd_total_payments[30m])

• The information is obtained in real time from lnd using the TrackPayments RPC.

After making payments between two directly connected nodes:

alice-64db66cb64-g4sh2:/# curl -s http://localhost:8990/metrics | grep lnd_total
# HELP lnd_total_htlc_attempts Total number of HTLC attempts across all payments
# TYPE lnd_total_htlc_attempts counter
lnd_total_htlc_attempts 5
# HELP lnd_total_payments Total number of payments initiated
# TYPE lnd_total_payments counter
lnd_total_payments 5

[calvinrzachman]

I may be able to consolidate the # attempts with the existing HTLC collector. Checking...