bugfix(op-node): fix the issue of inaccurate peerCount Metric

[welkin22]

Background

When we use DNS to specify the addresses of staticPeers, there may be a situation where one peer corresponds to multiple connections. Therefore, the same peer may trigger the Connected or the Disconnected event of Notifiee multiple times. This will cause inaccurate peerCount statistics. In addition, restarting the op-node service may also cause statistical problems. Because peerCount is a gauge type indicator, if the Disconnected event cannot be processed in time when the service is closed, peerCount may retain incorrect data and will not decrease to 0.

Solution

In order to minimize the impact of duplicate events and server restarts, we should use Gauge.Set(float64) instead of the currently used Inc*() and Dec*() methods. Getting the length of the peers array stored in the Network object each time will make the statistical indicators more accurate.