fix(metrics): protocol_state=None case not accounted for

[jansegre]

I recently got a traceback like this:

Traceback (most recent call last):
  File ".../python3.7/site-packages/twisted/internet/defer.py", line 311, in addCallbacks
    self._runCallbacks()
  File ".../python3.7/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File ".../python3.7/site-packages/twisted/internet/base.py", line 447, in _continueFiring
    callable(*args, **kwargs)
  File ".../python3.7/site-packages/twisted/internet/base.py", line 706, in disconnectAll
    failure.Failure(main.CONNECTION_LOST))
--- <exception caught here> ---
  File ".../python3.7/site-packages/twisted/python/log.py", line 103, in callWithLogger
    return callWithContext({"system": lp}, func, *args, **kw)
  File ".../python3.7/site-packages/twisted/python/log.py", line 86, in callWithContext
    return context.call({ILogContext: newCtx}, func, *args, **kw)
  File ".../python3.7/site-packages/twisted/python/context.py", line 122, in callWithContext
    return self.currentContext().callWithContext(ctx, func, *args, **kw)
  File ".../python3.7/site-packages/twisted/python/context.py", line 85, in callWithContext
    return func(*args,**kw)
  File ".../python3.7/site-packages/twisted/internet/tcp.py", line 519, in connectionLost
    self._commonConnection.connectionLost(self, reason)
  File ".../python3.7/site-packages/twisted/internet/tcp.py", line 327, in connectionLost
    protocol.connectionLost(reason)
  File ".../python3.7/site-packages/twisted/internet/endpoints.py", line 146, in connectionLost
    return self._wrappedProtocol.connectionLost(reason)
  File ".../python3.7/site-packages/twisted/protocols/tls.py", line 403, in connectionLost
    ProtocolWrapper.connectionLost(self, reason)
  File ".../python3.7/site-packages/twisted/protocols/policies.py", line 125, in connectionLost
    self.wrappedProtocol.connectionLost(reason)
  File ".../hathor-core/hathor/p2p/protocol.py", line 357, in connectionLost
    self.on_disconnect(reason)
  File ".../hathor-core/hathor/p2p/protocol.py", line 275, in on_disconnect
    self.connections.on_peer_disconnect(self)
  File ".../hathor-core/hathor/p2p/manager.py", line 220, in on_peer_disconnect
    self.pubsub.publish(HathorEvents.NETWORK_PEER_DISCONNECTED, protocol=protocol)
  File ".../hathor-core/hathor/pubsub.py", line 166, in publish
    fn(key, args)
  File ".../hathor-core/hathor/metrics.py", line 219, in handle_publish
    if data['protocol'].state.state_name == HathorProtocol.PeerState.READY.name:
builtins.AttributeError: 'NoneType' object has no attribute 'state_name'

It might not be too important because that happened when exiting hathor-core, but it's still an error. In the future we should consider a better typing strategy on the data variable that metric events receive because currently it erases the types of its values (by effectively being a Dict[str, Any]), so mypy can't see these kind of errors.

[codecov-commenter]

Codecov Report

Merging #223 (fd6e9b6) into dev (5e811b7) will decrease coverage by 0.02%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##              dev     #223      +/-   ##
==========================================
- Coverage   83.40%   83.38%   -0.03%     
==========================================
  Files         147      147              
  Lines       14107    14108       +1     
  Branches     1974     1974              
==========================================
- Hits        11766    11764       -2     
- Misses       1925     1927       +2     
- Partials      416      417       +1     

Impacted Files	Coverage Δ
hathor/metrics.py	84.28% <50.00%> (+0.11%)	⬆️
hathor/p2p/manager.py	69.78% <0.00%> (-1.28%)	⬇️

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