Complete the transformation to a middleware

[dbrattli]

• Use interface instead of single case DU for the observable like HTTP handler function.
• Rename NextAsync / ErrorAsync to OnNextAsync / OnErrorAsync
• Fix logging
• Refactor core logic into a generic middleware (that can be reused for other purposes)

[codecov[bot]]

Codecov Report

Merging #77 (890d9e2) into master (795811c) will increase coverage by 1.05%. The diff coverage is 78.71%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   77.88%   78.94%   +1.05%     
==========================================
  Files           8        9       +1     
  Lines         597      703     +106     
  Branches       47       14      -33     
==========================================
+ Hits          465      555      +90     
- Misses        104      141      +37     
+ Partials       28        7      -21     

Impacted Files	Coverage Δ
src/HttpHandler/Metrics.fs	100.00% <ø> (ø)
src/Middleware/Builder.fs	0.00% <0.00%> (ø)
src/HttpHandler/Logging.fs	70.58% <70.58%> (ø)
src/HttpHandler/HttpHandler.fs	73.78% <73.78%> (ø)
src/HttpHandler/HttpContext.fs	79.66% <79.16%> (ø)
src/HttpHandler/Fetch.fs	83.54% <83.54%> (ø)
src/HttpHandler/Builder.fs	88.88% <88.88%> (ø)
src/Middleware/Error.fs	89.58% <89.58%> (ø)
src/Middleware/Core.fs	90.69% <90.69%> (ø)
... and 6 more