gossip: export new inbound fee related gauges

[Roasbeef]

With the release of lnd v0.18.0 we have a new piece of data to export/graph/analyze: inbound fee discounts.

We should export two new gauges so we can track the uptake of this new feature on the network:

• A gauge for the total amount of channel updates that set the inbound fee to a non-zero value.
  • We can also check the code check for existence of the new TLV as well, so even those that have upgraded, but set it to zero would contribute to this metric.
• A histogram for the various values of the inbound fee discount. We can use this to create percentile graphs and heatmaps, etc.