Change prometheus selenium active sessions gauge to custom collector

[pearj]

• Now shows active test sessions by underlying proxy

Fixes #703

Description

Uses a custom prometheus collector to ensure the accuracy of active sessions. Additionally, active sessions are broken down by proxy type, meaning you could see active sessions by cloud proxy as well.

Motivation and Context

To fix the leaking of active session counts.

How Has This Been Tested?

Tested by hand.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)

Checklist:

• My code follows the code style of this project.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #705 into master will decrease coverage by 0.49%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##             master     #705     +/-   ##
===========================================
- Coverage     62.88%   62.38%   -0.5%     
+ Complexity      521      519      -2     
===========================================
  Files            45       46      +1     
  Lines          3255     3265     +10     
  Branches        283      282      -1     
===========================================
- Hits           2047     2037     -10     
- Misses         1015     1036     +21     
+ Partials        193      192      -1

[diemol]

Thanks @pearj!