fix(identify): only report observed address once per connection

[thomaseizinger]

Description

At the moment, libp2p-identify reports an observed address repeatedly as a new external address candidate even if it is the same address from the same connection. Unless the underlying transport supports roaming, a connection does not change its observed address.

We change the behaviour of libp2p-identify to remember the observed address for a particular connection and not re-emit the NewExternalAddrCandidate event for it. This allows users to probabilistically promote a candidate to an external address based on its report frequency. If an address is reported twice, it means we have two connections where the remote observed this address. Chances are, we have port-reuse enabled for this connection and it might thus be dialable or at least a good candidate for hole-punching.

Related: #4688.

Notes & open questions

cc @altonen

Change checklist

• I have performed a self-review of my own code
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• A changelog entry has been made in the appropriate crates

[thomaseizinger]

Applying internal-change because we just adjust the tests.

[mxinden]

Good to go from my end. Thanks.

[thomaseizinger]

@mxinden I updated some docs as well.

[mxinden]

Docs read well to me.

[mxinden]

Retriggered CI with "spurious network failure".