Add a prometheus metrics handler

[agxs]

No description provided.

[codecov]

Codecov Report

Merging #86 (c3e6b6b) into master (7f7d256) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #86      +/-   ##
==========================================
+ Coverage   78.47%   78.49%   +0.02%     
==========================================
  Files          44       44              
  Lines        2114     2116       +2     
==========================================
+ Hits         1659     1661       +2     
  Misses        455      455              

Impacted Files	Coverage Δ
nbexchange/app.py	66.23% <100.00%> (+0.44%)	⬆️

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