feat: add method to http_request_duration_seconds metric

[alcidesmig]

What does this do?

This adds the "method" label to http_request_duration_seconds metrics.

Why do we need it?

The difference between, e.g., GET /books and POST /books are big. In the rest APIs scenario, it becomes important to distinguish between duration metrics for different methods, and exporting the method should be the default behavior.

[codecov]

Codecov Report

Merging #251 (e5a3b4e) into master (a18d361) will not change coverage.
The diff coverage is 100.00%.

❗ Current head e5a3b4e differs from pull request most recent head c64308d. Consider uploading reports for the commit c64308d to get more accurate results

@@           Coverage Diff           @@
##           master     #251   +/-   ##
=======================================
  Coverage   95.68%   95.68%           
=======================================
  Files           5        5           
  Lines         348      348           
=======================================
  Hits          333      333           
  Misses         15       15           

Impacted Files	Coverage Δ
src/prometheus_fastapi_instrumentator/metrics.py	93.70% <100.00%> (ø)

[trallnag]

Thank you