Remove fmt.Println from Prometheus middleware

[gfloyd]

Description

Removes a stray fmt.Println from the Prometheus middleware.

Motivation and Context

With the Prometheus middleware enabled (using the Docker image with FLAGR_PROMETHEUS_ENABLED="true"), this is flooding our logs with float values like:

0.000139925
0.000169531
0.000258753
0.000200045
0.000197314
0.00013287
0.000159397
7.7382e-05
0.000159737
... etc

How Has This Been Tested?

Built and deployed a canary Docker image with this change, checked that the values were no longer being output to stdout.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #313 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   81.83%   81.88%   +0.05%     
==========================================
  Files          26       26              
  Lines        1536     1535       -1     
==========================================
  Hits         1257     1257              
+ Misses        210      209       -1     
  Partials       69       69

Impacted Files	Coverage Δ
pkg/config/middleware.go	64.66% <ø> (+0.48%)	⬆️

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[zhouzhuojie]

Thanks!