add Prometheus exporter.

[s-dwinter]

Add Prometheus exporter support.

Supports monitoring by Prometheus using globocom/tornado-prometheus exporter.

Monitoring is one of the essential elements to run in a product environment, so I think it is very useful to have it enabled by default.
Also, when running on Kubernetes, Prometheus monitoring is the most common.

[codecov-commenter]

Codecov Report

Merging #166 into master will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   94.06%   94.08%   +0.02%     
==========================================
  Files           6        8       +2     
  Lines         724      829     +105     
  Branches      120      141      +21     
==========================================
+ Hits          681      780      +99     
- Misses         29       34       +5     
- Partials       14       15       +1     

Impacted Files	Coverage Δ
webssh/main.py	70.27% <100.00%> (+2.62%)	⬆️
webssh/handler.py	96.44% <0.00%> (-0.47%)	⬇️
webssh/utils.py	95.95% <0.00%> (ø)
webssh/__init__.py	100.00% <0.00%> (ø)

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