Upgrade prometheus client to 1.11.1

[nicoloboschi]

The current version is vulnerable to http://web.nvd.nist.gov/view/vuln/detail?vulnId=CVE-2022-21698

[codecov-commenter]

Codecov Report

Merging #31 (be5ca85) into master (a66e4b1) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #31   +/-   ##
=======================================
  Coverage   12.98%   12.98%           
=======================================
  Files          17       17           
  Lines        1510     1510           
=======================================
  Hits          196      196           
  Misses       1300     1300           
  Partials       14       14           

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