middleware: add request context to Observer callback

[mccutchen]

Attempting to belatedly dogfood this Observer mechanism over in mccutchen/httpbingo.org#14, I realized that we should pass the request context to the observer callback so that tracing frameworks can propagate trace/span IDs and other custom per-request metadata.

Unfortunately, this is a breaking change, but I think it's worth the breakage to better support request tracing for those custom deployments that want an easy way to do it.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (25a9144) to head (1ea07a1).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #267      +/-   ##
==========================================
- Coverage   94.98%   94.84%   -0.14%     
==========================================
  Files           9        9              
  Lines        2032     2036       +4     
==========================================
+ Hits         1930     1931       +1     
- Misses         59       62       +3     
  Partials       43       43              

Files with missing lines	Coverage Δ
httpbin/middleware.go	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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