multi: add support for simple payments related metrics

[calvinrzachman]

Change Description

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 registration of the metrics is hidden behind the monitoring build tag as is the case for the existing gRPC related metrics.
• The structure was intended to allow reasonably easy extension if we want to allow application configuration to conditionally register some metrics but not others. You'd just need to add a flag similar to the existing PerfHistograms on the lncfg.Prometheus type and then conditionally indicate that the metric should not be registered via ShouldRegister. For now the payments related metrics are registered by default if the monitoring build tag is used.
• The counters are in-memory only so will reset to 0 after lnd restart. Despite this, they are still useful for tracking the throughput and other statistics. As a result they will not show the total # of payments made during the lifetime of the lightning node.

Questions

• I know that a lot of metric gathering has historically been off-loaded to lndmon. Is that a preferable pattern to continue here as well? I ask because incrementing counters in real-time is not possible via an external mechanism like lndmon which must query lnd for the information, likely via ListPayments RPC and will have to take care to count payments properly and not run into trouble if the paginated queries take longer than the scrape interval.

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[saubyk]

These can be used to compute and create visualizations for things like average payment throughput and average # of attempts per payment.

This sounds like something which should be offloaded to a reporting or data analysis tool. Are there similar metrics which are hidden behind the monitoring build tag?

Would this be something which can be a Faraday candidate? Wondering about the performance overhead which comes with it