MetricsMiddleware: Set hb_uri to NotFound for 404 errors

[CallumTodd7]

This PR modifies the MetricsMiddleware to always set a hb_uri dimension, even if the error is a 404. To keep with the spirit of #47, the dimension's value is set to "NotFound" instead of the uri, which would be an unbounded value.

Motivation

1. Prevents unbounded dimension values using the same method as File middleware should set endpoint if it hits a file #482; hard coding a string instead of dropping the dimension.
2. Works around an implementation limitation with swift-prometheus where the library detects when the dimension¹ set for a metric has changed and crashes the application with a fatalError.

Footnotes

1. aka label, in Prometheus' terminology ↩

[adam-fowler]

Looks good

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.01%. Comparing base (e78cde7) to head (45d2e78).
Report is 93 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #499      +/-   ##
==========================================
- Coverage   84.86%   83.01%   -1.86%     
==========================================
  Files          98       94       -4     
  Lines        5320     4280    -1040     
==========================================
- Hits         4515     3553     -962     
+ Misses        805      727      -78     

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