collectors: distinguish attempts by payment status

[calvinrzachman]

Add a label to the metric for tracking htlc attempts so that we can differentiate between attempts made as part of payments that are successful from attempts for payments which fail.

This allows you to answer questions like: How many attempts does a successful payment take on average?

The exported metrics would look like:

# HELP lnd_total_htlc_attempts Total number of HTLC attempts across all payments
# TYPE lnd_total_htlc_attempts counter
lnd_total_htlc_attempts{status="failed"} 10
lnd_total_htlc_attempts{status="succeeded"} 5

# HELP lnd_total_payments Total number of payments initiated, labeled by final status
# TYPE lnd_total_payments counter
lnd_total_payments{status="failed"} 1
lnd_total_payments{status="succeeded"} 5

Extends: #115

[djkazic]

Nice upgrade!

[bitromortac]

Nice, LGTM ⚡