feat(lightnode,libp2p): add prometheus metrics

[mrekucci]

Adds the following metrics:

• number of currently connected peers
• number of currently disconnected peers
• total number of kicked out peers

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This change is 

[acud]

Reviewed 4 of 4 files at r1.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @aloknerurkar, @anatollupacescu, and @mrekucci)

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pkg/topology/lightnode/container.go, line 63 at r1 (raw file):

func (c *Container) KickOut(_ p2p.Peer) {
	c.metrics.TotalKickedPeers.Inc()

this metric can be inside libp2p, it doesn't really matter to have it on the light node container. also, if the peer info is not used, why do we need it?

[mrekucci]

Reviewable status: 1 of 5 files reviewed, 1 unresolved discussion (waiting on @acud, @aloknerurkar, and @anatollupacescu)

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pkg/topology/lightnode/container.go, line 63 at r1 (raw file):

Previously, acud (acud) wrote…

this metric can be inside libp2p, it doesn't really matter to have it on the light node container. also, if the peer info is not used, why do we need it?

Done.

[acud]

Reviewed 4 of 4 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @aloknerurkar)