feat: Add promethues metrics to govppmux

[ondrej-fabry]

This PR contains:

• add prometheus metrics to govppmux and telemetry (more to come)
• fix parsing VPP version (strip zero bytes)
• added integration test case for parsing VPP version
• fix race condition when for concurrent stats retrievval from govppmux plugin
• update version package and some minor code cleanups

[codecov]

Codecov Report

Merging #1626 into master will increase coverage by 1.29%.
The diff coverage is 47.42%.

@@            Coverage Diff             @@
##           master    #1626      +/-   ##
==========================================
+ Coverage   57.72%   59.02%   +1.29%     
==========================================
  Files         486      284     -202     
  Lines       39721    23018   -16703     
==========================================
- Hits        22930    13586    -9344     
+ Misses      14384     8335    -6049     
+ Partials     2407     1097    -1310

Flag	Coverage Δ
#e2e	?
#unittests	59.02% <47.42%> (+0.03%)	⬆️