chore: add peer connecting event

[glevco]

Motivation

This PR is in preparation for PubSub changes in #694. The current implementation of the test_p2p_network_events() test,

hathor-core/tests/others/test_metrics.py

Lines 68 to 69 in 0dd129d

	# This will trigger sending to the pubsub one of the network events
	manager.connections.connect_to(endpoint, use_ssl=True)

Tries to start a connection, which fails because the reactor used in tests (Clock) doesn't support TCP connections. This fail is expected and triggers the emission of the PubSub NETWORK_PEER_CONNECTION_FAILURE event, which is what triggers the Metrics update, making the test work.

In #694, the Clock is getting changed to a MemoryReactorClock, which does support TCP connections (but doesn't complete them), so the NETWORK_PEER_CONNECTION_FAILURE is not triggered.

We add an almost effectless new NETWORK_PEER_CONNECTING event that gets triggered when the peers starts connecting, so the Metrics update is still triggered.

Acceptance Criteria

• Create new HathorEvents.NETWORK_PEER_CONNECTING event
  • Emit the event when a peer starts connecting to the p2p manager
  • Subscribe to the event and handle it in Metrics

Checklist

• If you are requesting a merge into master, confirm this code is production-ready and can be included in future releases as soon as it gets merged

[codecov]

Codecov Report

Merging #693 (02a3548) into master (0dd129d) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head 02a3548 differs from pull request most recent head 67e0848. Consider uploading reports for the commit 67e0848 to get more accurate results

@@            Coverage Diff             @@
##           master     #693      +/-   ##
==========================================
+ Coverage   84.10%   84.12%   +0.01%     
==========================================
  Files         246      246              
  Lines       20446    20448       +2     
  Branches     2759     2759              
==========================================
+ Hits        17197    17201       +4     
+ Misses       2674     2673       -1     
+ Partials      575      574       -1     

Impacted Files	Coverage Δ
hathor/metrics.py	86.30% <ø> (ø)
hathor/p2p/manager.py	72.58% <100.00%> (+0.06%)	⬆️
hathor/pubsub.py	96.00% <100.00%> (+0.05%)	⬆️

... and 1 file with indirect coverage changes