Remove metric locks, frame type and channel namespace resolution for transport messages sent/received

[FZambia]

Some improvements for Centrifuge metrics:

• Get rid of registry locks from all metric increments, this should remove contention and result into better latency overall
• Add transport_messages_sent_size, transport_messages_received, transport_messages_received_size
• Add frame type resolution to transport_messages_sent, transport_messages_sent_size, transport_messages_received, transport_messages_received_size
• Add possibility to annotate transport messages metrics (both sent and received) with channel namespace label. This is optional as may bring some overhead
• benchmark for incTransportMessagesSent, incTransportMessagesReceived (1 alloc may gone away in real code):

cpu: Intel(R) Core(TM) i7-9750H CPU @ 2.60GHz
BenchmarkTransportMessagesSent-12        	41522294	29.78 ns/op	  8 B/op	  1 allocs/op
BenchmarkTransportMessagesReceived-12    	39113955	30.66 ns/op	  8 B/op	  1 allocs/op

[codecov]

Codecov Report

Merging #325 (4616083) into master (c2d6d00) will decrease coverage by 0.21%.
Report is 2 commits behind head on master.
The diff coverage is 81.16%.

@@            Coverage Diff             @@
##           master     #325      +/-   ##
==========================================
- Coverage   84.04%   83.84%   -0.21%     
==========================================
  Files          38       38              
  Lines        7453     7471      +18     
==========================================
  Hits         6264     6264              
- Misses        886      893       +7     
- Partials      303      314      +11     

Files	Coverage Δ
config.go	100.00% <ø> (ø)
events.go	0.00% <ø> (ø)
handler_http_stream.go	65.35% <100.00%> (ø)
handler_sockjs.go	82.31% <100.00%> (ø)
handler_sse.go	71.03% <100.00%> (ø)
handler_websocket.go	83.38% <100.00%> (ø)
internal/queue/queue.go	100.00% <ø> (ø)
node.go	92.47% <92.10%> (-0.09%)	⬇️
client.go	80.92% <76.64%> (-0.33%)	⬇️
metrics.go	83.45% <81.67%> (-1.42%)	⬇️

... and 2 files with indirect coverage changes

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